The present invention is: a pearlitic steel rail excellent in wear resistance and ductility, characterized in that, in a steel rail having pearlite structure containing, in mass, 0.65 to 1.40% C, the number of the pearlite blocks having grain sizes in the range from 1 to 15 μm is 200 or more per 0.2 mm2 of an observation field at least in a part of the region down to a depth of 10 mm from the surface of the corners and top of the head portion; and a method for producing a pearlitic steel rail excellent in wear resistance and ductility, characterized by, in the hot rolling of said steel rail, applying finish rolling so that the temperature of the rail surface may be in the range from 850° C. to 1,000° C. and the sectional area reduction ratio at the final pass may be 6% or more, and then applying accelerated cooling to the head portion of said rail at a cooling rate in the range from 1 to 30° C./sec. from the austenite temperature range to at least 550° C.
|
4. A method of heat treatment for a pearlitic steel rail containing 0.65 to 1.40 mass % C and excellent in wear resistance and ductility, comprising:
applying finish hot rolling so that the temperature of the rail surface is in the range from 850° C. to 1,000° C. and the sectional area reduction ratio at the final pass is 6% or more;
applying accelerated cooling to the web portion of said steel rail at a cooling rate in the range from 2 to 20° C./sec. and to the head and base portions of said steel rail at a cooling rate in the range from 1 to 10° C./sec. from the austenite temperature range to a temperature not higher than 650° C., within 100 sec. after the finish hot rolling;
controlling the number of the pearlite blocks having grain sizes in the range from 1 to 15 μm so as to be 200 or more per 0.2 mm2 of observation field at least in a part of the region down to a depth of 10 mm from the surface of the corners and top of the head portion; and
reducing the amount of pro-eutectoid cementite structures forming in the web portion of the rail so that the number of the pro-eutectoid cementite network intersecting two line segments each 300 μm in length crossing each other at right angles (the number of intersecting pro-eutectoid cementite network, NC) at the center of the centerline in the web portion of the rail satisfies the expression NC<CE, wherein CE is defined by the following equation:
CE=60([mass % C])+10([mass % Si])+10([mass % Mn])+500([mass % P])+50([mass % S])+30([mass % Cr])+50. 7. A pearlitic steel rail excellent in wear resistance and ductility having pearlite structures containing, in mass, 0.65 to 1.40% C, 0.05 to 2.00% Si, and 0.05 to 2.00% Mn and the balance being Fe and unavoidable impurities, the number of the pearlite blocks having grain sizes in the range from 1 to 15 μm is 200 or more per 0.2 mm2 of observation field at least in a part of the region down to a depth of 10 mm from the surface of the corners and top of the head portion, the pearlitic steel rail being prepared by a method comprising:
finishing a continuous hot rolling the steel rail so that the temperature of the rail surface being in the range from 850° C. to 1000° C. and the sectional area reduction ratio at two or more passes being 1 to 30% per pass and the time period between the passes being 10 seconds or less and the sectional area reduction at the final pass being 6% or more;
applying accelerated cooling to the web portion of said steel rail at a cooling rate in the range from 2 to 20° C./sec. and to the head and base portions of said steel rail at a cooling rate in the range from 1 to 10° C./sec. from the austenite temperature range to a temperature not higher than 650° C., within 100 sec. after the hot rolling; and
reducing the amount of pro-eutectoid cementite structures forming in the web portion of the rail so that the number of the pro-eutectoid cementite network intersecting two line segments each 300 μm in length crossing each other at right angles (the number of intersecting pro-eutectoid cementite network, NC) at the center of the centerline in the web portion of the rail satisfies the expression NC≦CE, wherein CE is defined by the following equation:
CE=60([mass % C])+10([mass % Si])+10([mass % Mn])+500([mass % P])+50([mass % S])+30 ([mass % Cr])+50. 1. A method of heat treatment for a pearlitic steel rail containing 65 to 1.40 mass % C and excellent in wear resistance and ductility, comprising:
applying finish hot rolling so that the temperature of the rail surface is in the range from 850° C. to 1,000° C. and the sectional area reduction ratio at the final pass is 6% or more;
applying accelerated cooling to the web portion of said steel rail at a cooling rate in the range from 2 to 20° C./sec. and to the head and base portions of said steel rail at a cooling rate in the range from 1 to 10° C./sec, from the austenite temperature range to a temperature not higher than 650° C., within 100 sec. after the finish hot rolling;
controlling the number of the pearlite blocks having grain sizes in the range from 1 to 15 μm so as to be 200 or more per 0.2 mm2 of observation field at least in a part of the region down to a depth of 10 mm from the surface of the corners and top of the head portion; and
reducing the amount of pro-eutectoid cementite structures forming in the web portion of the rail so that the number of the pro-eutectoid cementite network intersecting two line segments each 300 μm in length crossing each other at right angles (the number of intersecting pro-eutectoid cementite network, NC) at the center of the centerline in the web portion of the rail satisfies the expression NC≦CE, wherein CE is defined by the following equation:
CE=60([mass % C])+10([mass % Si])+10([mass % Mn])+500([mass % P])+50([mass % S])+30([mass % Cr])+50, and wherein the method is further characterized in that, at the finish rolling in the hot rolling of said steel rail, continuous finish rolling is applied so that two or more rolling passes are applied at a sectional area reduction ratio of 1 to 30% per pass and the time period between the passes is 10 sec. or less.
2. The method of
3. The method of
5. The method of
6. The method of
8. The pearlitic steel rail excellent in wear resistance and ductility according to
9. The pearlitic steel rail excellent in wear resistance and ductility according to
10. The pearlitic steel rail excellent in wear resistance and ductility according to
11. The pearlitic steel rail excellent in wear resistance and ductility according to
12. The pearlitic steel rail excellent in wear resistance and ductility according to
13. The pearlitic steel rail excellent in wear resistance and ductility according to
14. The pearlitic steel rail excellent in wear resistance and ductility according to
15. The pearlitic steel rail excellent in wear resistance and ductility according to
|
|||||||||||||||||||||||||||||||||||||||||||||||||
This application is a Divisional of application Ser. No. 10/482,753 filed Dec. 29, 2003 now abandoned which is a 371 of PCT/JP03/04364 filed Apr. 4, 2003, incorporated by reference herein.
The present invention relates to: a pearlitic steel rail that is aimed at improving wear resistance at the head portion of a steel rail for a heavy-load railway, enhancing resistance to breakage of the rail by improving ductility through controlling the number of fine pearlite block grains at the head portion of the rail, and preventing the toughness of the web and base portions of the rail from deteriorating by reducing the formation of pro-eutectoid cementite structures at these portions; and a method for efficiently producing a high-quality pearlitic steel rail by optimizing the heating conditions of a bloom (slab) for said rail, thus preventing cracking and breakage during hot rolling, and suppressing decarburization in the outer surface layer of the bloom (slab).
Overseas, in heavy-load railways, attempts have been made to increase the speed and loading weight of a train to improve the efficiency of railway transportation. Such an improvement in the railway transportation efficiency means that the environment for the use of rails is becoming increasingly severe, and this requires further improvements in the material quality of rails. Specifically, wear at the gauge corner and the head side portions of a rail laid on a curved track increases drastically and the fact has come to be viewed as a problem from the viewpoint of the service life of a rail. In this background, the developments of rails aimed mainly at enhancing wear resistance have been promoted as described below.
1) A method of producing a high-strength rail having a tensile strength of 130 kgf/mm2 (1,274 MPa) or more, characterized by subjecting the head portion of the rail to accelerated cooling at a cooling rate of 1 to 4° C./sec. from the austenite temperature range to a temperature in the range from 850° C. to 500° C. after the end of rolling or the application of reheating (Japanese Unexamined Patent Publication No. S57-198216).
2) A rail excellent in wear resistance wherein a hyper-eutectoid steel (containing over 0.85 to 1.20% C) is used and the density of cementite in lamella in pearlite structures is increased (Japanese Unexamined Patent Publication No. H8-144016).
In the case 1) above, it is intended that high strength is secured by using a eutectoid carbon-containing steel (containing 0.7 to 0.8% C) and thus forming fine pearlite structures. However, there is a problem in that wear resistance is insufficient and rail breakage is likely to occur when the rail is used for a heavy load railway since ductility is low. In the case 2) above, it is intended that wear resistance is improved by using a hyper-eutectoid carbon steel (containing over 0.85 to 1.20% C), thus forming fine pearlite structures, and then increasing the density of cementite in lamellae in pearlite structures. However, ductility is prone to deteriorate and, therefore, resistance to breakage of a rail is low as the carbon content thereof is higher than that of a presently used eutectoid carbon-containing steel. Further, there is another problem in that segregation bands, where carbon and alloying elements are concentrated, are likely to form at the center portion of a casting at the stage of the cast of molten steel, pro-eutectoid cementite forms in a great amount along the segregation bands especially at the web portion, which is indicated by the reference numeral 5 in
In view of the above situation, the developments of rails have been promoted for solving the aforementioned problems as shown below.
3) A rail wherein a eutectoid steel (containing 0.60 to 0.85% C) is used, the average size of block grains in pearlite structures is made fine through rolling, and thus ductility and toughness are enhanced (Japanese Unexamined Patent Publication No. H8-109440).
4) A rail excellent in wear resistance wherein a hyper-eutectoid steel (containing over 0.85 to 1.20% C) is used, the density of cementite in lamella in pearlite structures is increased, and, at the same time, hardness is controlled (Japanese Unexamined Patent Publication No. H8-246100).
5) A rail excellent in wear resistance wherein a hyper-eutectoid steel (containing over 0.85 to 1.20% C) is used, the density of cementite in lamella in pearlite structures is increased, and, at the same time, hardness is controlled by applying a heat treatment to the head and/or web portion(s) (Japanese Unexamined Patent Publication No. H9-137228).
6) A rail wherein a hyper-eutectoid steel (containing over 0.85 to 1.20% C) is used, the average size of block grains in pearlite structures is made fine through rolling and, thus, ductility and toughness are enhanced (Japanese Unexamined Patent Publication No. H8-109439).
In the rails proposed in the cases 3) and 4) above, the wear resistance, ductility and toughness of pearlite structures are enhanced by making the average size of block grains in the pearlite structures fine, and the wear resistance of the pearlite structures is further enhanced by increasing a carbon content in a steel, increasing the density of cementite in lamellae in the pearlite structures and also increasing hardness. However, despite the proposed technologies, the ductility and toughness of rails have been insufficient in cold regions where the temperature falls below the freezing point. What is more, even when such average size of block grains in pearlite structures as described above is made still finer in an attempt to enhance the ductility and toughness of rails, it has been difficult to thoroughly suppress rail breakage in cold regions. Further, in the rails proposed in the cases 4) and 5) above, there is a problem in that, in some rolling lengths and rolling end temperatures of rails, the uniformity of the material quality of the rails in the longitudinal direction and the ductility of the head portions thereof cannot be secured. On top of that, although it is possible to secure the hardness of pearlite structures at head portions and suppress the formation of pro-eutectoid cementite structures at web portions by applying accelerated cooling to the head and web portions of rails, it has still been difficult to suppress the formation of pro-eutectoid cementite structures, which serve as the starting points of fatigue cracks and brittle cracks, at the base and base toe portions of the rails, even when the heat treatment methods disclosed above are employed. At a base toe portion in particular, as the sectional area is smaller than those at head and web portions, the temperature of a base toe portion at the end of rolling tends to be lower than those of the other portions and, as a result, pro-eutectoid cementite structures form before heat treatment. Furthermore, at a web portion too, there are still other problems in that: pro-eutectoid cementite structures are likely to form because the segregation bands of various alloying elements remain; and, additionally, the temperature of the web portion is low at the end of hot rolling. Therefore, an additional problem has been that it is impossible to completely prevent the fatigue cracks and brittle cracks originating at base toe and web portions.
What is more, in the rail disclosed in the case 6) above, though a technology of making the average size of block grains in pearlite structures fine in a hyper-eutectoid steel in an attempt to improve the ductility and toughness of a rail is disclosed, it has been difficult to thoroughly suppress the occurrence of rail breakage in cold regions.
In the aforementioned situation, a pearlitic steel rail excellent in wear resistance and ductility and a production method thereof are looked for, to make it possible, in a rail of pearlite structure having a high carbon content, to realize: a superior wear resistance at the head portion of the rail; a high resistance to rail breakage by enhancing ductility; the prevention of the formation of pro-eutectoid cementite structures by optimizing cooling conditions; and, in addition to those, the uniformity in material characteristics in the longitudinal direction of the rail and the suppression of decarburization at the outer surface of the rail.
The present invention provides a pearlitic steel rail excellent in wear resistance and ductility and a production method thereof, wherein, in a rail used for a heavy load railway, the wear resistance and ductility required of the railhead portion are enhanced, the resistance to rail breakage is improved in particular, and the fracture resistance of the web, base and base toe portions of the rail is improved by preventing pro-eutectoid cementite structures from forming.
Further, the present invention provides a high-efficiency and high-quality pearlitic steel rail, wherein: cracking and breakage during hot rolling are prevented by optimizing the maximum heating temperature and the retention time at a reheating process in the event of hot-rolling a high-carbon steel bloom (slab) for rail rolling; and, in addition, the deterioration of wear resistance and fatigue strength is suppressed by controlling decarburization in the outer surface layer of the rail.
Still further, the present invention provides a method for producing a pearlitic steel rail excellent in wear resistance and ductility, wherein, in a rail having a high carbon content, the occurrence of cracks caused by fatigue, brittleness and lack of toughness is prevented and, at the same time, the wear resistance of the head portion, the uniformity in material quality in the longitudinal direction of the rail and the ductility of the head portion of the rail are secured by applying accelerated cooling to the head, web and base portions of the rail immediately after the end of hot rolling or within a certain time period thereafter, further optimizing the selection of an accelerated cooling rate at the head portion, a rail length at rolling, and a temperature at the end of rolling, and, by so doing, suppressing the formation of pro-eutectoid cementite structures.
The gist of the present invention, that attains the above object, is as follows:
(1) A pearlitic steel rail excellent in wear resistance and ductility, characterized in that, in a steel rail having pearlite structures containing, in mass, 0.65 to 1.40% C, the number of the pearlite blocks having grain sizes in the range from 1 to 15 μm is 200 or more per 0.2 mm2 of observation field at least in a part of the region down to a depth of 10 mm from the surface of the corners and top of the head portion.
(2) A pearlitic steel rail excellent in wear resistance and ductility, characterized in that, in a steel rail having pearlite structures containing, in mass, 0.65 to 1.40% C, 0.05 to 2.00% Si, and 0.05 to 2.00% Mn, the number of the pearlite blocks having grain sizes in the range from 1 to 15 μm is 200 or more per 0.2 mm2 of observation field at least in a part of the region down to a depth of 10 mm from the surface of the corners and top of the head portion.
(3) A pearlitic steel rail excellent in wear resistance and ductility, characterized in that, in a steel rail having pearlite structures containing, in mass, 0.65 to 1.40% C, 0.05 to 2.00% Si, 0.05 to 2.00% Mn, and 0.05 to 2.00% Cr, the number of the pearlite blocks having grain sizes in the range from 1 to 15 μm is 200 or more per 0.2 mm2 of observation field at least in a part of the region down to a depth of 10 mm from the surface of the corners and top of the head portion.
(4) A pearlitic steel rail excellent in wear resistance and ductility according to any one of the items (1) to (3), characterized in that the C content of the steel rail is over 0.85 to 1.40%.
(5) A pearlitic steel rail excellent in wear resistance and ductility according to any one of the items (1) to (4), characterized in that the length of the rail after hot rolling is 100 to 200 m.
(6) A pearlitic steel rail excellent in wear resistance and ductility according to any one of the items (1) to (5), characterized in that the hardness in the region down to a depth of at least 20 mm from the surface of the corners and top of the head portion is in the range from 300 to 500 Hv.
(7) A pearlitic steel rail excellent in wear resistance and ductility according to any one of the items (1) to (6), characterized by further containing, in mass, 0.01 to 0.50% Mo.
(8) A pearlitic steel rail excellent in wear resistance and ductility according to any one of the items (1) to (7), characterized by further containing, in mass, one or more of 0.005 to 0.50% V, 0.002 to 0.050% Nb, 0.0001 to 0.0050% B, 0.10 to 2.00% Co, 0.05 to 1.00% Cu, 0.05 to 1.00% Ni, and 0.0040 to 0.0200% N.
(9) A pearlitic steel rail excellent in wear resistance and ductility according to any one of the items (1) to (8), characterized by further containing, in mass, one or more of 0.0050 to 0.0500% Ti, 0.0005 to 0.0200% Mg, 0.0005 to 0.0150% Ca, 0.0080 to 1.00% Al, and 0.0001 to 0.2000% Zr.
(10) A pearlitic steel rail excellent in wear resistance and ductility according to any one of the items (4) to (9), characterized by reducing the amount of pro-eutectoid cementite structures forming in the web portion of the rail so that the number of the pro-eutectoid cementite network intersecting two line segments each 300 μm in length crossing each other at right angles (the number of intersecting pro-eutectoid cementite network, NC) at the center of the centerline in the web portion of the rail may satisfy the expression NC≦CE in relation to the value of CE defined by the following equation (1):
CE=60([mass % C])+10([mass % Si])+10([mass % Mn])+500([mass % P])+50([mass % S])+30([mass % Cr])+50 (1).
(11) A method for producing a pearlitic steel rail excellent in wear resistance and ductility, characterized by, in the hot rolling of a steel rail containing 0.65 to 1.40 mass % C: applying finish rolling so that the temperature of the rail surface may be in the range from 850° C. to 1,000° C. and the sectional area reduction ratio at the final pass may be 6% or more; then applying accelerated cooling to the head portion of said rail at a cooling rate in the range from 1 to 30° C./sec. from the austenite temperature range to a temperature not higher than 550° C.; and controlling the number of the pearlite blocks having grain sizes in the range from 1 to 15 μm so as to be 200 or more per 0.2 mm2 of observation field at least in a part of the region down to a depth of 10 mm from the surface of the corners and top of the head portion.
(12) A method for producing a pearlitic steel rail excellent in wear resistance and ductility, characterized by, in the hot rolling of a steel rail containing, in mass, 0.65 to 1.40% C, 0.05 to 2.00% Si, and 0.05 to 2.00% Mn: applying finish rolling so that the temperature of the rail surface may be in the range from 850° C. to 1,000° C. and the sectional area reduction ratio at the final pass may be 6% or more; then applying accelerated cooling to the head portion of said rail at a cooling rate in the range from 1 to 30° C./sec. from the austenite temperature range to a temperature not higher than 550° C.; and controlling the number of the pearlite blocks having grain sizes in the range from 1 to 15 μm so as to be 200 or more per 0.2 mm2 of observation field at least in a part of the region down to a depth of 10 mm from the surface of the corners and top of the head portion.
(13) A method for producing a pearlitic steel rail excellent in wear resistance and ductility, characterized by, in the hot rolling of a steel rail containing, in mass, 0.65 to 1.40% C, 0.05 to 2.00% Si, 0.05 to 2.00% Mn, and 0.05 to 2.00% Cr: applying finish rolling so that the temperature of the rail surface may be in the range from 850° C. to 1,000° C. and the sectional area reduction ratio at the final pass may be 6% or more; then applying accelerated cooling to the head portion of said rail at a cooling rate in the range from 1 to 30° C./sec. from the austenite temperature range to a temperature not higher than 550° C.; and controlling the number of the pearlite blocks having grain sizes in the range from 1 to 15 μm so as to be 200 or more per 0.2 mm2 of observation field at least in a part of the region down to a depth of 10 mm from the surface of the corners and top of the head portion.
(14) A method for producing a pearlitic steel rail excellent in wear resistance and ductility according to any one of the items (11) to (13), characterized in that, at the finish rolling in the hot rolling of said steel rail, continuous finish rolling is applied so that two or more rolling passes may be applied at a sectional area reduction ratio of 1 to 30% per pass and the time period between the passes may be 10 sec. or less.
(15) A method for producing a pearlitic steel rail excellent in wear resistance and ductility according to any one of the items (11) to (13), characterized by applying accelerated cooling to the head portion of said rail at a cooling rate in the range from 1 to 30° C./sec. from the austenite temperature range to a temperature not higher than 550° C. within 200 sec. after the end of the finish rolling in the hot rolling of said steel rail.
(16) A method for producing a pearlitic steel rail excellent in wear resistance and ductility according to any one of the items (11) to (13), characterized by applying accelerated cooling within 200 sec. after the end of the finish rolling in the hot rolling of said steel rail: to the head portion of said rail at a cooling rate in the range from 1 to 30° C./sec. from the austenite temperature range to a temperature not higher than 550° C.; and to the web and base portions of said rail at a cooling rate in the range from 1 to 10° C./sec. from the austenite temperature range to a temperature not higher than 650° C.
(17) A method for producing a pearlitic steel rail excellent in wear resistance and ductility according to any one of the items (11) to (16), characterized by, in a reheating process for a bloom or slab containing aforementioned steel composition, reheating said bloom or slab so that: the maximum heating temperature (Tmax, ° C.) of said bloom or slab may satisfy the expression Tmax≦CT in relation to the value of CT defined by the following equation (2) composed of the carbon content of said bloom or slab; and the retention time (Mmax, min.) of said bloom or slab after said bloom or slab is heated to a temperature of 1,100° C. or above may satisfy the expression Mmax≦CM in relation to the value of CM defined by the following equation (3) composed of the carbon content of said bloom or slab:
CT=1,500−140([mass % C])−80([mass % C])2 (2),
CM=600−120([mass % C])−60([mass % C])2 (3).
(18) A method for producing a pearlitic steel rail excellent in wear resistance and ductility according to any one of the items (11) to (16), characterized by applying accelerated cooling, after hot-rolling a bloom or slab containing aforementioned steel composition into the shape of a rail: within 60 sec. after the hot rolling, to the base toe portions of said steel rail at a cooling rate in the range from 5 to 20° C./sec. from the austenite temperature range to a temperature not higher than 650° C.; and to the head, web and base portions of said steel rail at a cooling rate in the range from 1 to 10° C./sec. from the austenite temperature range to a temperature not higher than 650° C.
(19) A method for producing a pearlitic steel rail excellent in wear resistance and ductility according to any one of the items (11) to (16), characterized by applying accelerated cooling, after hot-rolling a bloom or slab containing aforementioned steel composition into the shape of a rail: within 100 sec. after the hot rolling, to the web portion of said steel rail at a cooling rate in the range from 2 to 20° C./sec. from the austenite temperature range to a temperature not higher than 650° C.; and to the head and base portions of said steel rail at a cooling rate in the range from 1 to 10° C./sec. from the austenite temperature range to a temperature not higher than 650° C.
(20) A method for producing a pearlitic steel rail excellent in wear resistance and ductility according to any one of the items (11) to (16), characterized by applying accelerated cooling, after hot-rolling a bloom or slab containing aforementioned steel composition into the shape of a rail: within 60 sec. after the hot rolling, to the base toe portions of said steel rail at a cooling rate in the range from 5 to 20° C./sec. from the austenite temperature range to a temperature not higher than 650° C.; within 100 sec. after the hot rolling, to the web portion of said steel rail at a cooling rate in the range from 2 to 20° C./sec. from the austenite temperature range to a temperature not higher than 650° C.; and to the head and base portions of said steel rail at a cooling rate in the range from 1 to 10° C./sec. from the austenite temperature range to a temperature not higher than 650° C.
(21) A method for producing a pearlitic steel rail excellent in wear resistance and ductility according to any one of the items (11) to (16), characterized by, after hot-rolling a bloom or slab containing aforementioned steel composition into the shape of a rail: within 60 sec. after the hot rolling, raising the temperature at the base toe portions of said steel rail to a temperature 50° C. to 100° C. higher than the temperature before the temperature rising; and also applying accelerated cooling to the head, web and base portions of said steel rail at a cooling rate in the range from 1 to 10° C./sec. from the austenite temperature range to a temperature not higher than 650° C.
(22) A method for producing a pearlitic steel rail excellent in wear resistance and ductility according to any one of the items (11) to (16), characterized by, after hot-rolling a bloom or slab containing aforementioned steel composition into the shape of a rail: within 100 sec. after the hot rolling, raising the temperature at the web portion of said steel rail to a temperature 20° C. to 100° C. higher than the temperature before the temperature rising; and also applying accelerated cooling to the head, web and base portions of said steel rail at a cooling rate in the range from 1 to 10° C./sec. from the austenite temperature range to a temperature not higher than 650° C.
(23) A method for producing a pearlitic steel rail excellent in wear resistance and ductility according to any one of the items (11) to (16), characterized by, after hot-rolling a bloom or slab containing aforementioned steel composition into the shape of a rail: within 60 sec. after the hot rolling, raising the temperature at the base toe portions of said steel rail to a temperature 20° C. to 100° C. higher than the temperature before the temperature rising; within 100 sec. after the hot rolling, raising the temperature at the web portion of said steel rail to a temperature 20° C. to 100° C. higher than the temperature before the temperature rising; and also applying accelerated cooling to the head, web and base portions of said steel rail at a cooling rate in the range from 1 to 10° C./sec. from the austenite temperature range to a temperature not higher than 650° C.
(24) A method for producing a pearlitic steel rail excellent in wear resistance and ductility according to any one of the items (11) to (16), characterized by, in the event of acceleratedly cooling the head portion of said steel rail from the austenite temperature range, applying the accelerated cooling so that the cooling rate (ICR, ° C./sec.) in the temperature range from 750° C. to 650° C. at a head inner portion 30 mm in depth from the head top surface of said steel rail may satisfy the expression ICR≧CCR in relation to the value of CCR defined by the following equation (4) composed of the chemical compositions of said steel rail:
CCR=0.6+10×([% C]−0.9)−5×([% C]−0.9)×[% Si]−0.17[% Mn]−0.13[% Cr] (4).
(25) A method for producing a pearlitic steel rail excellent in wear resistance and ductility according to any one of the items (11) to (16), characterized by, in the event of acceleratedly cooling the head portion of said steel rail from the austenite temperature range, applying the accelerated cooling so that the value of TCR defined by the following equation (5) composed of the respective cooling rates in the temperature range from 750° C. to 500° C. at the surfaces of the head top portion (TH, ° C./sec.), the head side portions (TS, ° C./sec.) and the lower chin portions (TJ, ° C./sec.) of said steel rail may satisfy the expression 4CCR≧TCR≧2CCR in relation to the value of CCR defined by the following equation (4) composed of the chemical compositions of said steel rail:
CCR=0.6+10×([% C]−0.9)−5×([% C]−0.9)×[% Si]−0.17[% Mn]−0.13[% Cr] (4),
TCR=0.05TH(° C./sec.)+0.10TS(° C./sec.)+0.50TJ(° C./sec.) (5).
(26) A method for producing a pearlitic steel rail excellent in wear resistance and ductility according to any one of the items (11) to (25), characterized in that the C content of the steel rail is 0.85 to 1.40%.
(27) A method for producing a pearlitic steel rail excellent in wear resistance and ductility according to any one of the items (11) to (26), characterized in that the length of the rail after hot rolling is 100 to 200 m.
(28) A method for producing a pearlitic steel rail excellent in wear resistance and ductility according to any one of the items (11) to (27), characterized in that the hardness in the region down to a depth of at least 20 mm from the surface of the corners and top of the head portion of a pearlitic steel rail according to any one of the items (1) to (10) is in the range from 300 to 500 Hv.
(29) A method for producing a pearlitic steel rail excellent in wear resistance and ductility according to any one of the items (11) to (28), characterized in that the steel rail further contains, in mass, 0.01 to 0.50% Mo.
(30) A method for producing a pearlitic steel rail excellent in wear resistance and ductility according to any one of the items (11) to (29), characterized in that the steel rail further contains, in mass, one or more of 0.005 to 0.50% V, 0.002 to 0.050% Nb, 0.0001 to 0.0050% B, 0.10 to 2.00% Co, 0.05 to 1.00% Cu, 0.05 to 1.00% Ni, and 0.0040 to 0.0200% N.
(31) A method for producing a pearlitic steel rail excellent in wear resistance and ductility according to any one of the items (11) to (30), characterized in that the steel rail further contains, in mass, one or more of 0.0050 to 0.0500% Ti, 0.0005 to 0.0200% Mg, 0.0005 to 0.0150% Ca, 0.0080 to 1.00% Al, and 0.0001 to 0.2000% Zr.
(32) A method for producing a pearlitic steel rail excellent in wear resistance and ductility according to any one of the items (11) to (31), characterized by reducing the amount of pro-eutectoid cementite structures forming in the web portion of the rail so that the number of the pro-eutectoid cementite network intersecting two line segments each 300 μm in length crossing each other at right angles (the number of intersecting pro-eutectoid cementite network, NC) at the center of the centerline in the web portion of the rail may satisfy the expression NC ≦CE in relation to the value of CE defined by the following equation (1):
CE=60([mass % C])+10([mass % Si])+10([mass % Mn])+500([mass % P])+50([mass % S])+30([mass % Cr])+50 (1).
The present invention is hereafter explained in detail.
The present inventors studied, in the first place, the relationship between the occurrence of rail breakage and the mechanical properties of pearlite structures. As a result, it has been confirmed that the occurrence of the rail breakage originating from the railhead portion correlates well with ductility evaluated in a tensile test rather than toughness evaluated in an impact test, in which a loading speed is comparatively high, because the loading speed imposed on the railhead portion by contact with a wheel is comparatively low.
Then the present inventors re-examined the relationship between ductility and the block size of pearlite structures in a steel rail of pearlite structures having a high carbon content. As a result, it has been confirmed that, though the ductility of pearlite structures tends to improve as the average size of block grains in the pearlite structures decreases, the ductility does not improve sufficiently with the mere decrease in the average size of the block grains in a region where the average size of the block grains is very fine.
In view of this, the present inventors studied dominating factor of the ductility of pearlite structures in a region where the average size of the block grains in pearlite structures was very fine. As a result, it has been discovered that the ductility of pearlite structures correlates not with the average block grain size but with the number of the fine pearlite block grains having certain grain sizes and that the ductility of pearlite structures significantly improves by controlling the number of the fine pearlite block grains having certain grain sizes to a certain value or more in a given area of a visual field.
On the basis of the above findings, the present inventors have discovered that, in a steel rail of pearlite structures having a high carbon content, both the wear resistance and the ductility at the railhead portion are improved simultaneously by controlling the number of the fine pearlite block grains having certain grain sizes in the railhead portion.
That is, an object of the present invention is, in a high-carbon containing rail for heavy load railways, to enhance the wear resistance at the head portion thereof, and, at the same time, to prevent the occurrence of fracture such as breakage of the rail by improving ductility through the control of the number of the fine pearlite block grains having certain grain sizes.
Next, the reasons for regulating the conditions in the present invention are hereafter explained in detail.
(1) Regulations for the Size and the Number of Pearlite Block Grains
Firstly, the reasons are explained for regulating the size of pearlite block grains, the size being used for regulating the number of the pearlite block grains, in the range from 1 to 15 μm.
A pearlite block having a grain size larger than 15 μm does not significantly contribute to improving the ductility of fine pearlite structures. On the other hand, though a pearlite block having a grain size smaller than 1 μm contributes to improving the ductility of fine pearlite structures, the contribution thereof is insignificant. For those reasons, the size of pearlite block grains, the size being used for regulating the number of the pearlite block grains, is regulated in the range from 1 to 15 μm.
Secondly, the reasons are explained for regulating the number of the pearlite block grains having grain sizes in the range from 1 to 15 μm to 200 or more per 0.2 mm2 of observation field.
When the number of the pearlite block grains having grain sizes in the range from 1 to 15 μm is less than 200 per 0.2 mm2 of observation field, it becomes impossible to improve the ductility of fine pearlite structures. No upper limit is particularly set forth with regard to the number of the pearlite block grains having grain sizes in the range from 1 to 15 μm, but, from restrictions on the rolling temperature during hot rolling and the cooling conditions during heat treatment in rail production, 1,000 grains per 0.2 mm2 of observation field is the upper limit, substantially.
Thirdly, the reasons are explained for specifying that the region, in which the number of the pearlite block grains having grain sizes in the range from 1 to 15 μm is determined to be 200 or more per 0.2 mm2 of observation field, is at least a part of the region down to a depth of 10 mm from the surface of the corners and top of a head portion.
The rail breakage that originates from a railhead portion begins, basically, from the surface of the head portion. For this reason, in order to prevent rail breakage, it is necessary to enhance the ductility of the surface layer of a railhead portion, namely, to increase the number of the pearlite block grains having grain sizes in the range from 1 to 15 μm. As a result of experimentally examining the correlation between the ductility of the surface layer of a railhead portion and the pearlite blocks in the surface layer thereof, it has been clarified that the ductility of the surface layer of a railhead portion correlates with the pearlite block size in the region down to a depth of 10 mm from the surface of the head top portion. In addition, as a result of further examining the correlation between the ductility of the surface layer of a railhead portion and the pearlite blocks in the surface layer thereof, it has been confirmed that the ductility of the surface layer of the railhead portion is improved and, consequently, the rail breakage is inhibited as long as a region where the number of the pearlite block grains having grain sizes in the range from 1 to 15 μm is 200 or more exists at least in a part of the aforementioned region. The above regulations are determined on the basis of the results from the aforementioned examinations.
Here, the method of measuring the size of pearlite block grains is described. Methods of measuring pearlite block grains include (i) the modified curling etch method, (ii) the etch pit method, and (iii) the electron back-scatter diffraction pattern (EBSP) method wherein an SEM is used. In the above examinations, since the size of the pearlite block grains was fine, it was difficult to confirm the size by the modified curling etch method (i) or the etch pit method (ii), and, therefore, the EBSP method (iii) was employed.
The conditions of the measurement are described hereafter. The measurement of the size of pearlite block grains followed the conditions and procedures described in the items (ii) to (vii) below, and the number of the pearlite block grains having grain sizes in the range from 1 to 15 μm per 0.2 mm2 of observation field was counted. The measurement was done at least in two observation fields at each of observation positions, the number of the grains in each of the observation fields was counted according to the following procedures, and the average of the numbers of the grains in two or more observation fields was used as the value representing an observation position.
The reasons are explained in detail for regulating the chemical composition of a steel rail in the ranges specified in the claims.
C is an element effective for accelerating pearlitic transformation and securing wear resistance. If the amount of C is 0.65% or less, then a sufficient hardness of pearlite structures in a railhead portion cannot be secured, in addition pro-eutectoid ferrite structures form, therefore wear resistance deteriorates, and, as a result, the service life of the rail is shortened. If the amount of C exceeds 1.40%, on the other hand, then pro-eutectoid cementite structures form in pearlite structures at the surface layer and the inside of a railhead and/or the density of cementite phases in the pearlite structures increases, and thus the ductility of the pearlite structures deteriorates. In addition, the number of intersecting pro-eutectoid cementite network (NC) in the web portion of a rail increases and the toughness of the web portion deteriorates. For those reasons, the amount of C is limited in the range from 0.65 to 1.40%. Note that, for enhancing wear resistance still more, it is desirable to set the amount of C to over 0.85% by which the density of cementite phases in pearlite structures can increase still more and thus wear resistance can further be enhanced.
Si is a component indispensable as a deoxidizing agent. Also, Si is an element that increases the hardness (strength) of a railhead portion by the solid solution hardening effect of Si in a ferrite phase in pearlite structures and, at the same time, improves the hardness and toughness of the rail by inhibiting the formation of pro-eutectoid cementite structures. However, if the content of Si is less than 0.05%, then these effects are not expected sufficiently, and no tangible improvement in hardness and toughness is obtained. If the content of Si exceeds 2.00%, on the other hand, then surface defects occur in a great deal during hot rolling and/or weldability deteriorates caused by the formation of oxides. Besides, in that case, pearlite structures themselves become brittle, thus not only the ductility of a rail deteriorates but also surface damage such as spalling occurs and, therefore, the service life of the rail shortens. For those reasons, the amount of Si is limited in the range from 0.05 to 2.00%.
Mn is an element that enhances hardenability, secures the hardness of pearlite structures by decreasing the pearlite lamella spacing, and thus improves wear resistance. However, if the content of Mn is less than 0.05%, then the effects are insignificant and it becomes difficult to secure the wear resistance required of a rail. If the content of Mn is more than 2.00%, on the other hand, then hardenability is increased remarkably, therefore martensite structures detrimental to wear resistance and toughness tend to form, and segregation is accelerated. What is more, in a high-carbon steel (C>0.85%) in particular, pro-eutectoid cementite structures form in the web and other portions, the number of intersecting pro-eutectoid cementite network (NC) increases in the web portion, and thus the toughness of a rail deteriorates. For those reasons, the amount of Mn is limited in the range from 0.05 to 2.00%.
Note that, for inhibiting the formation of pro-eutectoid cementite structures in the web portion of a rail, it is necessary to regulate the addition amounts of P and S. For that purpose, it is desirable to control their addition amounts within the respective ranges specified below for the following reasons.
P is an element that strengthens ferrite and enhances the hardness of pearlite structures. However, since P is an element that easily causes segregation, if the content of P exceeds 0.030%, it also accelerates the segregation of other elements and, as a result, the formation of pro-eutectoid cementite structures in a web portion is significantly accelerated. Consequently, the number of intersecting pro-eutectoid cementite network (NC) in the web portion of a rail increases and the toughness of the web portion deteriorates. For those reasons, the amount of P is limited to 0.030% or less.
S is an element that contributes to the acceleration of pearlitic transformation by generating MnS and forming Mn-depleted zone around the MnS and is effective for enhancing the toughness of pearlite structures by making the size of pearlite blocks fine as a result of the above contribution. However, if the content of S exceeds 0.025%, the segregation of Mn is accelerated and, as a result, the formation of pro-eutectoid cementite structures in a web portion is violently accelerated. Consequently, the number of intersecting pro-eutectoid cementite network (NC) in the web portion of a rail increases and the toughness of the web portion deteriorates. For those reasons, the amount of S is limited to 0.025% or less.
Further, the elements of Cr, Mo, V, Nb, B, Co, Cu, Ni, Ti, Mg, Ca, Al and Zr may be added, as required, to a steel rail having the chemical composition specified above for the purposes of: enhancing wear resistance by strengthening pearlite structures; preventing the deterioration of toughness by inhibiting the formation of pro-eutectoid cementite structures; preventing the softening and embrittlement of a weld heat-affected zone; improving the ductility and toughness of pearlite structures; strengthening pearlite structures; preventing the formation of pro-eutectoid cementite structures; and controlling the hardness distribution in the cross sections of the head portion and the inside of a rail.
Among those elements, Cr and Mo secure the hardness of pearlite structures by raising the equilibrium transformation temperature of pearlite and, in particular, by decreasing the pearlite lamella spacing. V and Nb inhibit the growth of austenite grains by forming carbides and nitrides during hot rolling and subsequent cooling and, in addition, improve the ductility and hardness of pearlite structures by precipitation hardening. Further, they stably form carbides and nitrides during reheating and thus prevent the heat-affected zones of weld joints from softening. B reduces the dependency of a pearlitic transformation temperature on a cooling rate and uniformalizes the hardness distribution in a railhead portion. Co and Cu dissolve in ferrite in pearlite structures and thus increase the hardness of the pearlite structures. Ni prevents embrittlement caused by the addition of Cu during hot rolling, increases the hardness of a pearlitic steel at the same time, and, in addition, prevents the heat-affected zones of weld joints from softening.
Ti makes the structure of a heat-affected zone fine and prevents the embrittlement of a weld joint. Mg and Ca make austenite grains fine during the rolling of a rail, accelerate pearlitic transformation at the same time, and improve the ductility of pearlite structures. Al strengthens pearlite structures and suppresses the formation of pro-eutectoid cementite structure by shifting a eutectoid transformation temperature toward a higher temperature and, at the same time, a eutectoid carbon concentration toward a higher carbon, and thus enhances the wear resistance of a rail and prevents the toughness thereof from deteriorating. Zr forms ZrO2 inclusions, which serve as solidification nuclei in a high-carbon steel rail, and thus increases an equi-axed crystal grain ratio in a solidification structure. As a result, it suppresses the formation of segregation bands at the center portion of a casting and the formation of pro-eutectoid cementite structures detrimental to the toughness of a rail. The main object of N addition is to enhance toughness by accelerating pearlitic transformation originating from austenite grain boundaries and making pearlite structures fine.
The reasons for regulating each of the aforementioned chemical compositions are hereunder explained in detail.
Cr is an element that contributes to the hardening (strengthening) of pearlite structures by raising the equilibrium transformation temperature of pearlite and consequently making the pearlite structures fine, and, at the same time, enhances the hardness (strength) of the pearlite structures by strengthening cementite phases. If the content of Cr is less than 0.05%, however, the effects are insignificant and the effect of enhancing the hardness of a steel rail does not show. If Cr is excessively added in excess of 2.00%, on the other hand, then hardenability increases, martensite structures form in a great amount, and the toughness of a rail deteriorates. In addition, segregation is accelerated, the amount of pro-eutectoid cementite structures forming in a web portion increases, consequently the number of intersecting pro-eutectoid cementite network (NC) increases, and therefore the toughness of the web portion of a rail deteriorates. For those reasons, the amount of Cr is limited in the range from 0.05 to 2.00%.
Mo, like Cr, is an element that contributes to the hardening (strengthening) of pearlite structures by raising the equilibrium transformation temperature of pearlite and consequently narrowing the space between adjacent pearlite lamellae and enhances the hardness (strength) of pearlite structures as a result. If the content of Mo is less than 0.01%, however, the effects are insignificant and the effect of enhancing the hardness of a steel rail does not show at all. If Mo is excessively added in excess of 0.50%, on the other hand, then the transformation rate of pearlite structures is lowered significantly, and martensite structures detrimental to toughness are likely to form. For those reasons, the addition amount of Mo is limited in the range from 0.01 to 0.50%.
V is an element effective for: making austenite grains fine by the pinning effect of v carbides and v nitrides when heat treatment for heating a steel material to a high temperature is applied; further enhancing the hardness (strength) of pearlite structures by the precipitation hardening of V carbides and V nitrides that form during cooling after hot rolling; and, at the same time, improving ductility. V is also an element effective for preventing the heat-affected zone of a weld joint from softening by forming v carbides and v nitrides in a comparatively high temperature range at a heat-affected zone reheated to a temperature in the range of not higher than the Ac1 transformation temperature. If the content of V is less than 0.005%, however, the effects are not expected sufficiently and the enhancement of the hardness of pearlite structures and the improvement of the ductility thereof are not realized. If V is added in excess of 0.500%, on the other hand, then coarse V carbides and v nitrides form, and the toughness and the resistance to internal fatigue damage of a rail deteriorate. For those reasons, the amount of V is limited in the range from 0.005 to 0.500%.
Nb, like V, is an element effective for: making austenite grains fine by the pinning effect of Nb carbides and Nb nitrides when heat treatment for heating a steel material to a high temperature is applied; further enhancing the hardness (strength) of pearlite structures by the precipitation hardening of Nb carbides and Nb nitrides that form during cooling after hot rolling; and, at the same time, improving ductility. Nb is also an element effective for preventing the heat-affected zone of a welded joint from softening by forming Nb carbides and Nb nitrides stably in the temperature range from a low temperature to a high temperature at a heat-affected zone reheated to a temperature in the range of not higher than the Ac1 transformation temperature. If the content of Nb is less than 0.002%, however, the effects are not expected and the enhancement of the hardness of pearlite structures and the improvement of the ductility thereof are not realized. If Nb is added in excess of 0.050%, on the other hand, then coarse Nb carbides and Nb nitrides form, and the toughness and the resistance to internal fatigue damage of a rail deteriorate. For those reasons, the amount of Nb is limited in the range from 0.002 to 0.050%.
B is an element that suppresses the formation of pro-eutectoid cementite by forming carbo-borides of iron, uniformalizes the hardness distribution in a head portion at the same time by lowering the dependency of a pearlitic transformation temperature on a cooling rate, prevents the deterioration of the toughness of a rail, and extends the service life of the rail as a result. If the content of B is less than 0.0001%, however, the effects are insufficient and no improvement in the hardness distribution in a railhead portion is realized. If B is added in excess of 0.0050%, on the other hand, then coarse carbo-borides of iron form, and ductility, toughness and resistance to internal fatigue damage are significantly deteriorated. For those reasons, the amount of B is limited in the range from 0.0001 to 0.0050%.
Co is an element that dissolves in ferrite in pearlite structures and enhances the hardness (strength) of the pearlite structures by solid solution strengthening. Co is also an element that improves ductility by increasing the transformation energy of pearlite and making pearlite structures fine. If the content of Co is less than 0.10%, however, the effects are not expected. If Co is added in excess of 2.00%, on the other hand, then the ductility of ferrite phases deteriorates significantly, spalling damage occurs at a wheel rolling surface, and resistance to the surface damage of a rail deteriorates. For those reasons, the amount of Co is limited in the range from 0.10 to 2.00%.
Cu is an element that dissolves in ferrite in pearlite structures and enhances the hardness (strength) of the pearlite structures by solid solution strengthening. If the content of Cu is less than 0.05%, however, the effects are not expected. If Cu is added in excess of 1.00%, on the other hand, then hardenability is enhanced remarkably and, as a result, martensite structures detrimental to toughness are likely to form. In addition, in that case, the ductility of ferrite phases is significantly lowered and therefore the ductility of a rail deteriorates. For those reasons, the amount of Cu is limited in the range from 0.05 to 1.00%.
Ni is an element that prevents embrittlement caused by the addition of Cu during hot rolling and, at the same time, hardens (strengthens) a pearlitic steel through solid solution strengthening by dissolving in ferrite. In addition, Ni is an element that, at a weld heat-affected zone, precipitates as the fine grains of the intermetallic compounds of Ni3Ti in combination with Ti and inhibits the softening of the weld heat-affected zone by precipitation strengthening. If the content of Ni is less than 0.01%, however, the effects are very small. If Ni is added in excess of 1.00%, on the other hand, the ductility of ferrite phases is lowered significantly, spalling damage occurs at a wheel rolling surface, and resistance to the surface damage of a rail deteriorates. For those reasons, the amount of Ni is limited in the range from 0.01 to 1.00%.
Ti is an element effective for preventing the embrittlement of the heat-affected zone of a weld joint by taking advantage of the fact that carbides and nitrides of Ti having precipitated during the reheating of the weld joint do not dissolve again and thus making fine the structure of the heat-affected zone heated to a temperature in the austenite temperature range. If the content of Ti is less than 0.0050%, however, the effects are insignificant. If Ti is added in excess of 0.0500%, on the other hand, then coarse carbides and nitrides of Ti form and the ductility, toughness and resistance to internal fatigue damage of a rail deteriorate significantly. For those reasons, the amount of Ti is limited in the range from 0.0050 to 0.0500%.
Mg is an element effective for improving the ductility of pearlite structures by forming fine oxides in combination with O, S, Al and so on, suppressing the growth of crystal grains during reheating for the rolling of a rail, and thus making austenite grains fine. In addition, MgO and MgS make MnS disperse in fine grains, thus form Mn-depleted zone around the MnS, and contribute to the progress of pearlitic transformation. Therefore, Mg is an element effective for improving the ductility of pearlite structures by making a pearlite block size fine. If the content of Mg is less than 0.0005%, however, the effects are insignificant. If Mg is added in excess of 0.0200%, on the other hand, then coarse oxides of Mg form and the toughness and resistance to internal fatigue damage of a rail deteriorate. For those reasons, the amount of Mg is limited in the range from 0.0005 to 0.0200%.
Ca has a strong bonding power with S and forms sulfides in the form of CaS. Further, CaS makes MnS disperse in fine grains and thus forms Mn-depleted zone around the MnS. Therefore, Ca contributes to the progress of pearlitic transformation and, as a result, is an element effective for improving the ductility of pearlite structures by making a pearlite block size fine. If the content of Ca is less than 0.0005%, however, the effects are insignificant. If Ca is added in excess of 0.0150%, on the other hand, then coarse oxides of Ca form and the toughness and resistance to internal fatigue damage of a rail deteriorate. For those reasons, the amount of Ca is limited in the range from 0.0005 to 0.0150%.
Al is an element that shifts a eutectoid transformation temperature toward a higher temperature and, at the same time, a eutectoid carbon concentration toward a higher carbon. Thus, Al is an element that strengthens pearlite structures and prevents the deterioration of toughness, by inhibiting the formation of pro-eutectoid cementite structures. If the content of Al is less than 0.0080%, however, the effects are insignificant. If Al is added in excess of 1.00%, on the other hand, it becomes difficult to make Al dissolve in a steel, thus coarse alumina inclusion serving as the origins of fatigue damage form, and consequently the toughness and resistance to internal fatigue damage of a rail deteriorate. In addition, in that case, oxides form during welding and weldability is remarkably deteriorated. For those reasons, the amount of Al is limited in the range from 0.0080 to 1.00%.
Zr is an element that functions as the solidification nuclei in a high-carbon steel rail in which γ-Fe is the primary crystal of solidification, because ZrO2 inclusions have good lattice coherent with γ-Fe, thus increases an equi-axed crystal ratio in a solidification structure, by so doing, inhibits the formation of segregation bands at the center portion of a casting, and suppresses the formation of pro-eutectoid cementite structures detrimental to the toughness of a rail. If the amount of Zr is less than 0.0001%, however, then the number of ZrO2 inclusions is so small that their function as the solidification nuclei does not bear a tangible effect, and, as a consequence, the effect of suppressing the formation of pro-eutectoid cementite structures is reduced. If the amount of Zr exceeds 0.2000%, on the other hand, then coarse Zr inclusions form in a great amount, thus the toughness of a rail deteriorates, internal fatigue damage originating from coarse Zr system inclusions is likely to occur, and, as a result, the service life of the rail shortens. For those reasons, the amount of Zr is limited in the range from 0.0001 to 0.2000%.
N accelerates the pearlitic transformation originating from austenite grain boundaries by segregating at the austenite grain boundaries, and thus makes the pearlite block size fine. Therefore, N is an element effective for enhancing the toughness and ductility of pearlite structures. If the content of N is less than 0.0040%, however, the effects are insignificant. If N is added in excess of 0.0200%, on the other hand, it becomes difficult to make N dissolve in a steel and gas holes functioning as the origins of fatigue damage form in the inside of a rail. For those reasons, the amount of N is limited in the range from 0.0040 to 0.0200%.
A steel rail that has such chemical composition as described above is melted and refined in a commonly used melting furnace such as a converter or an electric arc furnace, then resulting molten steel is processed through ingot casting and breakdown rolling or continuous casting, and thereafter the resulting casting is produced into rails through hot rolling. Subsequently, accelerated cooling is applied to the head portion of a hot-rolled rail maintaining the high temperature heat at the hot rolling or being reheated to a high temperature for the purpose of heat treatment, and, by so doing, pearlite structures having a high hardness can be stably formed in the railhead portion.
As a method for controlling the number of the pearlite blocks having grain sizes in the range from 1 to 15 μm so as to be 200 or more per 0.2 mm2 of observation field at least in a part of the region down to a depth of 10 mm from the surface of the corners and top of a railhead portion in the above production processes, a method desirable satisfies the conditions of: setting the temperature during hot rolling as low as possible; applying accelerated cooling as quickly as possible after the rolling; by so doing, suppressing the growth of austenite grains immediately after rolling; and raising an area reduction ratio at the final rolling so that the accelerated cooling may be applied while high strain energy is accumulated in the austenite grains. Desirable hot rolling and heat treatment conditions are as follows: a final rolling temperature is 980° C. or lower; an area reduction ratio at the final rolling is 6% or more; and an accelerated cooling rate is 1° C./sec. or more in average of range from the austenite temperature range to 550° C.
Further, in the case where a rail is reheated for the purpose of heat treatment, as it is impossible to make use of the effect of strain energy, it is desirable to set a reheating temperature as low as possible and an accelerated cooling rate as high as possible. Desirable conditions of heat treatment for reheating are as follows: a reheating temperature is 1,000° C. or lower; and an accelerated cooling rate is 5° C./sec. or more in average of range from the austenite temperature range to 550° C.
(3) Hardness of a Railhead Portion and the Range of the Hardness
Here, the reasons are explained for regulating the hardness in the region down to a depth of 20 mm from the surface of the corners and top of a railhead portion so as to be in the range from 300 to 500 Hv.
In a steel having chemical composition according to the present invention, if hardness is below 300 Hv, then it becomes difficult to secure a good wear resistance and the service life of a rail shortens. If hardness exceeds 500 Hv, on the other hand, resistance to surface damage is significantly deteriorated as a result of: the accumulation of fatigue damage at a wheel rolling surface caused by an extravagant improve in wear resistance; and/or the occurrence of rolling fatigue damage such as dark spot damage caused by the development of a crystallographic texture. For those reasons, the hardness of pearlite structures is limited in the range from 300 to 500 in Hv.
Next, the reasons are explained for regulating the portion, where the hardness is regulated in the range from 300 to 500 Hv, so as to be in the region down to a depth of 20 mm from the surface of the corners and top of a head portion.
If the depth of the portion where the hardness is regulated in the range from 300 to 500 Hv is less than 20 mm, then, in consideration of the service life of a rail, the depth of the portion where the wear resistance required of a rail must be secured is insufficient and it becomes difficult to secure a sufficiently long service life of the rail. If the portion where the hardness is regulated in the range from 300 to 500 Hv extends down to a depth of 30 mm or more from the surface of the corners and top of a head portion, the rail service life is further extended, which is more desirable.
In relation to the above,
Therefore, it is desirable that pearlite structures having hardness controlled within the above range are located in the vicinity of the surface of a railhead portion that mainly contacts with wheels, and the other portions may consist of any metallographic structures other than a pearlite structure.
Next, the present inventors quantified the amount of pro-eutectoid cementite structures forming in the web portion of a rail. As a result of measuring the number of the pro-eutectoid cementite network intersecting two line segments of a prescribed length crossing each other at right angles (hereinafter referred to as the number of intersecting pro-eutectoid cementite network, NC) in an observation field under a prescribed magnification, a good correlation has been found between the number of intersecting pro-eutectoid cementite network and the state of cementite structure formation, and it has been clarified that the state of pro-eutectoid cementite structure formation can be quantified on the basis of the correlation.
Subsequently, the present inventors investigated the relationship between the toughness of a web portion and the state of pro-eutectoid cementite structure formation using steel rails of pearlite structures having a high carbon content. As a result, it has been clarified that, in a steel rail of pearlite structures having a high carbon content: (i) the toughness of the web portion of the rail is in negative correlation with the number of intersecting pro-eutectoid cementite network (NC); (ii) if the number of intersecting pro-eutectoid cementite network (NC) is not more than a certain value, then the toughness of the web portion does not deteriorate; and (iii) the threshold value of the number of intersecting pro-eutectoid cementite network (NC) beyond which the toughness deteriorates correlates with the chemical compositions of the steel rail.
On the basis of the above findings, the present inventors tried to clarify the relationship between the threshold value of the number of intersecting pro-eutectoid cementite network (NC) beyond which the toughness of the web portion of a rail deteriorated, and the chemical compositions of the steel rail, by using multiple correlation analysis. As a result, it has been found that the threshold value of the number of intersecting pro-eutectoid cementite network (NC) beyond which the toughness of a web portion decreases can be defined by the value (CE) calculated from the following equation (1) that evaluates the contributions of chemical compositions (in mass %) in a steel rail.
Further, the present inventors studied a means for improving the toughness of the web portion of a rail. As a result, it has been found that the amount of pro-eutectoid cementite structures forming in the web portion of a rail is reduced to a level lower than that of a presently used steel rail and the toughness of the web portion of the rail is prevented from deteriorating by controlling the number of intersecting pro-eutectoid cementite network (NC) in the web portion of the rail so as to be not more than the value of CE calculated from the chemical composition of the rail:
CE=60[mass % C]−10[mass % Si]+10[mass % Mn]+500[mass % P]+50[mass % S]+30[mass % Cr]−54 (1),
Note that, in the present invention, in order to reduce the number of intersecting pro-eutectoid cementite network (NC) at the center of the centerline in the web portion of a rail, it is effective: with regard to continuous casting, (i) to optimize the soft reduction by a means such as the control of a casting speed and (ii) to make a solidification structure fine by lowering the temperature of casting; and, with regard to the heat treatment of a rail, (iii) to apply accelerated cooling to the web portion of a rail in addition to the head portion thereof. In order to reduce the number of intersecting pro-eutectoid cementite network (NC) still further, it is effective: to combine the above measures in continuous casting and heat treatment; to add Al, which has an effect of suppressing the formation of pro-eutectoid cementite structures; and/or to add Zr, which makes a solidification structure fine.
(4) Method for Exposing Pro-Eutectoid Cementite Structures in the Web Portion of a Rail
The method for exposing pro-eutectoid cementite structures is explained hereunder. Firstly, a cross-sectional surface of the web portion of a rail is polished with diamond abrasive, subsequently, the polished surface is immersed in a solution of picric acid and caustic soda, and thus pro-eutectoid cementite structures are exposed. Some adjustments may be required of the exposing conditions in accordance with the condition of a polished surface, but, basically, desirable exposing conditions are: an immersion solution temperature is 80° C.; and an immersion time is approximately 120 min.
(5) Method for Measuring the Number of Intersecting Pro-Eutectoid Cementite Network (NC)
Next, the method for measuring the number of intersecting pro-eutectoid cementite network (NC) is explained. Pro-eutectoid cementite is likely to form at the boundaries of prior austenite crystal grains. The portion where pro-eutectoid cementite structures are exposed at the center of the centerline on a sectional surface of the web portion of a rail is observed with an optical microscope. Then, the number of intersections (expressed in the round marks in
(6) Equation for Calculating the Value of CE
Here, the reason is explained for defining the equation for calculating the value of CE as described earlier. The equation for calculating the value of CE has been obtained, using steel rails of pearlite structures having a high carbon content, by taking the procedures of: investigating the relationship between the toughness of a web portion and the state of pro-eutectoid cementite structure formation; and then clarifying the relationship between the threshold value of the number of intersecting pro-eutectoid cementite network (NC) beyond which the toughness of the web portion deteriorates and the chemical composition (in mass %) of the steel rail by using multiple correlation analysis. The resulting correlation equation (1) is shown below:
CE=60[mass % C]−10[mass % Si]+10[mass % Mn]+500[mass % P]+50[mass % S]+30[mass % Cr]−54 (1).
The coefficient affixed to the content of each of the constituent chemical composition represents the contribution of the relevant component to the formation of cementite structures in the web portion of a rail, and the sign + means that the relevant component has a positive correlation with the formation of cementite structures, and the sign − a negative correlation. The absolute value of each of the coefficients represents the magnitude of the contribution. A value of CE is defined as an integer of the value calculated from the equation above, round up numbers of five and above and drop anything under five. Note that, in some combinations of the chemical composition specified in the above equation, the value of CE may be 0 or negative. Such a case that the value of CE is 0 or negative is regarded as outside of the scope of the present invention, even if the contents of the chemical composition conform to the relevant ranges specified earlier.
In addition, the present inventors examined the causes for generating cracks in a bloom (slab) having a high carbon content in the processes of reheating and hot rolling the casting into rails. As a result, it has been clarified that: some parts of a casting are melted at segregated portions in solidification structures in the vicinity of the outer surface of the casting where the heating temperature of the casting is the highest; the melted parts burst by the subsequent rolling; and thus cracks are generated. It has also been clarified that, the higher the maximum heating temperature of a casting is or the higher the carbon content of a casting is, the more the cracks tend to be generated.
On the basis of the above findings, the present inventors experimentally studied the relationship between the maximum heating temperature of a casting at which melted parts that caused cracks were generated and the carbon content in the casting. As a result, it has been found that the maximum heating temperature of a casting at which the melted parts are generated can be regulated by a quadratic expression which is shown as the following equation (2) composed of the carbon content (in mass %) of the casting, and that the melted parts of a casting in a reheated state and accompanying cracks or breaks during hot rolling can be prevented by controlling the maximum heating temperature (Tmax, ° C.) of the casting to not more than the value of CT calculated from the quadratic equation:
CT=1500−140([mass % C])−80([mass % C])2 (2).
Next, the present inventors analyzed the factors that accelerated the decarburization in the outer surface layer of the bloom (slab) having a high carbon content in a reheating process for hot rolling the bloom (slab) into rails. As a result, it has been clarified that the decarburization in the outer surface layer of the bloom (slab) is significantly influenced by a temperature and a retention time in the reheating of the casting and moreover the carbon content in the bloom (slab).
On the basis of the above findings, the present inventors studied the relationship among a temperature and a retention time in the reheating of the bloom (slab), a carbon content in the bloom (slab), and the amount of decarburization in the outer surface layer of the bloom (slab). As a result, it has been found that, the longer the retention time at a temperature not lower than a certain temperature is and the higher the carbon content in the bloom (slab) is, the more the decarburization in the outer surface layer of the bloom (slab) is accelerated.
In addition, the present inventors experimentally studied the relationship between the carbon content in the bloom (slab) and a retention time in the reheating of the bloom (slab) that does not cause the deterioration of the properties of a rail after final rolling. As a result, it has been found that, when a reheating temperature is 1,100° C. or higher, the retention time of the bloom (slab) can be regulated by a quadratic expression which is shown as the following equation (3) composed of the carbon content (in mass %) of the bloom (slab), and that the decrease of the carbon content and the deterioration of hardness in pearlite structures in the outer surface layer of the bloom (slab) can be suppressed and also the deterioration of the wear resistance and the fatigue strength of a rail after final rolling can be suppressed by controlling the reheating time of the bloom (slab) (Mmax, min.) to not more than the value of CM calculated from the quadratic equation:
CM=600−120([mass % C])−60([mass % C])2 (3).
As stated above, the present inventors have found that, by optimizing the maximum heating temperature of the bloom (slab) having a high carbon content and the retention time thereof at a heating temperature not lower than a certain temperature in a reheating process for hot rolling the bloom (slab) into rails: the partial melting of the bloom (slab) is prevented and thus cracks and breaks are prevented during hot rolling; further the decarburization in the outer surface layer of a rail is inhibited and thus the deterioration of wear resistance and fatigue strength is suppressed; and, as a consequence, a high quality rail can be produced efficiently.
In other words, the present invention makes it possible to efficiently produce a high quality rail by preventing the partial melting of the bloom (slab) having a high carbon content and suppressing the decarburization in the outer surface layer of the bloom (slab) in a reheating process for hot rolling the bloom (slab) into rails. The conditions specified in the present invention are explained hereunder.
(7) Reasons for Limiting the Maximum Heating Temperature (Tmax, ° C.) of a Bloom (Slab) in a Reheating Process for Hot Rolling
Here, the reasons are explained in detail for limiting the maximum heating temperature (Tmax, ° C.) of a bloom (slab) to not more than the value of CT calculated from the carbon content of a steel rail in a reheating process for hot rolling the bloom (slab) into rails.
The present inventors experimentally investigated the factors that caused partial melting to occur in a bloom (slab) having a high carbon content in a reheating process for hot rolling the bloom (slab) into rails and thus cracks to be generated in the bloom (slab) during hot rolling. As a result, it has been confirmed that, the higher the maximum heating temperature of a bloom (slab) is and the higher the carbon content thereof is, partial melting is apt to occur in the bloom (slab) during reheating and cracks are apt to be generated during hot rolling.
On the basis of the findings, the present inventors tried to find the relationship between the carbon content of a bloom (slab) and the maximum heating temperature thereof beyond which partial melting occurred in the bloom (slab) by using multiple correlation analysis. The resulting correlation equation (2) is shown below:
CT=1500−140([mass % C])−80([mass % C])2 (2).
As stated above, the equation (2) is an experimental regression equation, and partial melting in a bloom (slab) during reheating and accompanying cracks and breaks during rolling can be prevented by controlling the maximum heating temperature (Tmax, ° C.) of the bloom (slab) to not more than the value of CT calculated from the quadratic equation composed of the carbon content of the bloom (slab).
(8) Reasons for Limiting the Retention Time (Mmax, min.) of a Bloom (Slab) in a Reheating Process for Hot Rolling
Here, the reasons are explained in detail for limiting the retention time (Mmax, min.) of a bloom (slab) heated to a temperature of 1,100° C. or higher in a reheating process for hot rolling the bloom (slab) into rails to not more than the value of CM calculated from the carbon content of a steel rail.
The present inventors experimentally investigated the factors that increased the amount of decarburization in the outer surface layer of a bloom (slab) having a high carbon content in a reheating process for hot rolling the bloom (slab) into rails. As a result, it has been clarified that, the longer the retention time at a temperature not lower than a certain temperature is and the higher the carbon content in a bloom (slab) is, the more the decarburization is accelerated during reheating.
On the basis of the findings, the present inventors tried to find out the relationship, in the reheating temperature range of 1,100° C. or higher where the decarburization of a casting was significant, between the carbon content of a bloom (slab) and the retention time of the bloom (slab) beyond which the properties of a rail after final rolling deteriorated by using multiple correlation analysis. The resulting correlation equation (3) is shown below:
CM=600−120([mass % C])−60([mass % C])2 (3).
As stated above, the equation (3) is an experimental regression equation, and the decrease in the carbon content and the hardness of pearlite structures in the outer surface layer of a bloom (slab) is inhibited and thus the deterioration of the wear resistance and the fatigue strength of a rail after final rolling is suppressed by controlling the retention time (Mmax, min.) of the bloom (slab) in the reheating temperature range of 1,100° C. or higher to not more than the value of CM calculated from the quadratic equation.
Note that no lower limit is particularly specified for a retention time (Mmax, min.) in the reheating of a bloom (slab), but it is desirable to control a retention time to 250 min. or longer from the viewpoint of heating a casting sufficiently and uniformly and securing formability at the time of the rolling of a rail.
With regard to the control of the temperature and the time of reheating as specified above in a reheating process for hot rolling a bloom (slab) into rails, it is desirable to directly measure a temperature at the outer surface of a bloom (slab) and to control the temperature thus obtained and the time. However, when the measurement is difficult industrially, by controlling the average temperature of the atmosphere in a reheating furnace and the resident time in the furnace in a prescribed temperature range of the furnace atmosphere too, similar effects can be obtained and a high-quality rail can be produced efficiently.
Next, the present inventors studied a heat treatment method capable of, in a steel rail having a high carbon content, enhancing the hardness of pearlite structures in the railhead portion and suppressing the formation of pro-eutectoid cementite structures in the web and base portions thereof. As a result, it has been confirmed that, with regard to a rail after hot rolling, it is possible to enhance the hardness of the railhead portion and suppress the formation of pro-eutectoid cementite structures in the web and base portions thereof by applying accelerated cooling to the head portion and also another accelerated cooling to the web and base portions either from the austenite temperature range within a prescribed time after rolling or after the rail is heated again to a certain temperature.
As the first step of the above studies, the present inventors studied a method for hardening pearlite structures in a railhead portion in commercial rail production. As a result, it has been found that: the hardness of pearlite structures in a railhead portion correlates with the time period from the end of hot rolling to the beginning of the subsequent accelerated cooling and the rate of the accelerated cooling; and it is possible to form pearlite structures in a railhead portion and harden the portion by controlling the time period after the end of hot rolling and the rate of subsequent accelerated cooling within respective prescribed ranges and further by controlling the temperature at the end of the accelerated cooling to not lower than a prescribed temperature.
As the second step, the present inventors studied a method that makes it possible to suppress the formation of pro-eutectoid cementite structures in the web and base portions of a rail in commercial rail production. As a result, it has been found that: the formation of pro-eutectoid cementite structures correlates with the time period from the end of hot rolling to the beginning of the subsequent accelerated cooling and the conditions of the accelerated cooling; and it is possible to suppress the formation of pro-eutectoid cementite structures by controlling the time period after the end of hot rolling within a prescribed range and further by either (i) controlling the accelerated cooling rate within a prescribed range and the accelerated cooling end temperature to not lower than a prescribed temperature, or (ii) applying heating up to a temperature within a prescribed temperature range and thereafter controlling the accelerated cooling rate within a prescribed range.
In addition to the above production methods, the present inventors studied a rail production method for securing the uniformity of the material quality of a rail in the longitudinal direction in the above production methods. As a result, it has been clarified that, when the length of a rail at hot rolling exceeds a certain length: the temperature difference between the two ends of the rail and the middle portion thereof and moreover between the ends of the rail after the rolling is excessive; and, by the above-mentioned rail production method, it is difficult to control the temperature and the cooling rate over the whole length of the rail and thus the material quality of the rail in the longitudinal direction becomes uneven. Then, the present inventors studied an optimum rolling length of a rail for securing the uniformity of the material quality of the rail through the test rolling of real rails. As a result, it has been found that a certain adequate range exists in the rolling length of a rail in consideration of economical efficiency.
In addition, the present inventors studied a rail production method for securing the ductility of a railhead portion. As a result, it has been found that: the ductility of a railhead portion correlates with the temperature and the area reduction ratio of hot rolling, the time period between rolling passes and the time period from the end of final rolling to the beginning of heat treatment; and it is possible to secure both the ductility of a railhead portion and the formability of a rail at the same time by controlling the temperature of the railhead portion at final rolling, the area reduction ratio, the time period between rolling passes and the time period to the beginning of heat treatment within respective prescribed ranges.
As stated above, in the present invention, it has been found that, with regard to a steel rail having a high carbon content: it is possible to harden the railhead portion and thus secure the wear resistance of the railhead portion and to suppress the formation of pro-eutectoid cementite structures at the web and base portions of the rail, the structures being detrimental to the fatigue cracking and brittle fracture, by applying accelerated cooling to the head, web and base portions of the rail within a prescribed time period after the end of hot rolling and, in addition, by applying another accelerated cooling to the web and base toe portions of the rail after the rail is heated; and further it is possible to secure the wear resistance of the railhead portion, the uniformity of the material quality of the rail in the longitudinal direction, the ductility of the railhead portion, and the fatigue strength and fracture toughness of the web and base portions of the rail by optimizing the length of the rail at rolling, the temperature of the railhead portion at final rolling, the area reduction ratio, the time period between rolling passes, and the time period from the end of rolling to the beginning of heat treatment.
In other words, the present invention makes it possible to, in a steel rail having a high carbon content: make the size of pearlite blocks fine; secure the ductility of the railhead portion; prevent the deterioration of the wear resistance of the railhead portion and the fatigue strength and fracture toughness of the web and base portions of the rail; and secure the uniformity of the material quality of the rail in the longitudinal direction.
(9) Reasons for Limiting the Conditions of Accelerated Cooling
Here, the reasons are explained in detail for limiting the time period from the end of hot rolling to the beginning of accelerated cooling, and the rate and the temperature range of accelerated cooling.
In the first place, explanations are given regarding the time period from the end of hot rolling to the beginning of accelerated cooling.
When the time period from the end of hot rolling to the beginning of accelerated cooling exceeds 200 sec., with the chemical composition according to the present invention, austenite grains coarsen after rolling, as a consequence pearlite blocks coarsen, and ductility is not improved sufficiently, and, with some chemical composition according to the present invention, pro-eutectoid cementite structures form and the fatigue strength and toughness of a rail deteriorate. For those reasons, the time period from the end of hot rolling to the beginning of accelerated cooling is limited to not longer than 200 sec. Note that, even if the time period exceeds 200 sec., the material quality of a rail is not significantly deteriorated except for ductility. Therefore, as far as the time period is not longer than 250 sec., a rail quality acceptable for actual use can be secured.
Meanwhile, in a section of a rail immediately after the end of hot rolling, an uneven temperature distribution exists caused by heat removal by rolling rolls during rolling and so on, and, as a result, material quality in the rail section becomes uneven after accelerated cooling. In order to suppress temperature unevenness in a rail section and uniformalize material quality in the rail section, it is desirable to begin accelerated cooling after the lapse of not less than 5 sec. from the end of the rolling.
Next, explanations are given regarding the range of an accelerated cooling rate.
First, the conditions of accelerated cooling at a railhead portion are explained. When the accelerated cooling rate of a railhead portion is below 1° C./sec., with the chemical composition according to the present invention, the railhead portion cannot be hardened and it becomes difficult to secure the wear resistance of the railhead portion. In addition, pro-eutectoid cementite structures form and the ductility of the rail deteriorates. What is more, the pearlitic transformation temperature rises, pearlite blocks coarsen, and the ductility of the rail deteriorates. When an accelerated cooling rate exceeds 30° C./sec., on the other hand, with the chemical composition according to the present invention, martensite structures form and the toughness of a railhead portion deteriorates significantly. For those reasons, the accelerated cooling rate of a railhead portion is limited in the range from 1 to 30° C./sec.
Note that the accelerated cooling rate mentioned above is not a cooling rate during cooling but an average cooling rate from the beginning to the end of accelerated cooling. Therefore, as far as an average cooling rate from the beginning to the end of accelerated cooling is within the range specified above, it is possible to make a pearlite block size fine and simultaneously harden a railhead portion.
Next, explanations are given regarding the temperature range of accelerated cooling. When accelerated cooling at a railhead portion is finished at a temperature above 550° C., an excessive thermal recuperation takes place from the inside of a rail after the end of the accelerated cooling. As a result, the pearlitic transformation temperature is pushed up by the temperature rise and it becomes impossible to harden pearlite structures and secure a good wear resistance. In addition, pearlite blocks coarsen and the ductility of the rail deteriorates. For those reasons, the present invention stipulates that accelerated cooling should be applied until the temperature reaches a temperature not higher than 550° C.
No lower limit is particularly specified for the temperature at which accelerated cooling at a railhead portion is finished but, for securing a good hardness at a railhead portion and preventing the formation of martensite structures which are likely to form at segregated portions and the like in a head inner portion, 400° C. is the lower limit temperature, substantially.
Second, explanations are given regarding the conditions of accelerated cooling at the head, web and base portions of a rail, for preventing the formation of pro-eutectoid cementite structures.
In the first place, the range of an accelerated cooling rate is explained. When an accelerated cooling rate is below 1° C./sec., with the chemical composition according to the present invention, it becomes difficult to prevent the formation of pro-eutectoid cementite structures. When an accelerated cooling rate exceeds 10° C./sec., on the other hand, with the chemical composition according to the present invention, martensite structures form at segregated portions in the web and base portions of a rail and the toughness of the rail significantly deteriorates. For those reasons, an accelerated cooling rate is limited in the range from 1 to 10° C./sec.
Note that the accelerated cooling rate mentioned above is not a cooling rate during cooling but an average cooling rate from the beginning to the end of accelerated cooling. Therefore, as far as an average cooling rate from the beginning to the end of accelerated cooling is within the range specified above, it is possible to suppress the formation of pro-eutectoid cementite structures.
Next, explanations are given regarding the temperature range of accelerated cooling. When accelerated cooling is finished at a temperature above 650° C., an excessive thermal recuperation takes place from the inside of a rail after the end of the accelerated cooling. As a result, pearlite structures are prevented from forming by the temperature rise and, instead, pro-eutectoid cementite structures form. For these reasons, the present invention stipulates that accelerated cooling should be applied until the temperature reaches a temperature not higher than 650° C.
No lower limit is practically specified for the temperature at which accelerated cooling is finished but, for suppressing the formation of pro-eutectoid cementite structures and preventing the formation of martensite structures at the segregated portions in a web portion, 500° C. is the lower limit temperature, substantially.
(10) Reasons for Limiting the Heat Treatment Conditions of the Web and Base Portions of a Rail
For the purpose of thoroughly preventing the formation of pro-eutectoid cementite structures in the web and base toe portions of a rail, a restrictive heat treatment is applied in addition to the cooling explained above. Here, the conditions of the heat treatment of the web and base toe portions of a rail are explained.
First, the conditions of the heat treatment of the web portion of a rail are explained. Explanations begin with the time period from the end of hot rolling to the beginning of accelerated cooling at the web portion of a rail. When the time period from the end of hot rolling to the beginning of accelerated cooling at the web portion of a rail exceeds 100 sec., with the chemical composition according to the present invention, pro-eutectoid cementite structures form in the web portion of the rail before the accelerated cooling and the fatigue strength and toughness of the rail deteriorate. For those reasons, the time period till the beginning of accelerated cooling is limited to not longer than 100 sec.
No lower limit is particularly specified for the time period from the end of hot rolling to the beginning of accelerated cooling at the web portion of a rail but, to make uniform the size of austenite grains in the web portion of a rail and mitigating the temperature unevenness occurring during rolling, it is desirable to begin accelerated cooling after the lapse of not less than 5 sec. from the end of hot rolling.
Next, explanations are given regarding the range of the cooling rate of accelerated cooling at the web portion of a rail. When a cooling rate is below 2° C./sec., with the chemical composition according to the present invention, it becomes difficult to prevent the formation of pro-eutectoid cementite structures in the web portion of a rail. When a cooling rate exceeds 20° C./sec., on the other hand, with the chemical composition according to the present invention, martensite structures form at the segregation bands in the web portion of a rail and the toughness of the web portion of the rail significantly deteriorates. For those reasons, an accelerated cooling rate at the web portion of a rail is limited in the range from 2 to 20° C./sec.
Note that the accelerated cooling rate at the web portion of a rail mentioned above is not a cooling rate during cooling but an average cooling rate from the beginning to the end of accelerated cooling. Therefore, as long as an average cooling rate from the beginning to the end of accelerated cooling is within the range specified above, it is possible to suppress the formation of pro-eutectoid cementite structures.
Next, explanations are given regarding the temperature range of accelerated cooling at the web portion of a rail. When accelerated cooling is finished at a temperature above 650° C., an excessive thermal recuperation takes place from the inside of a rail after the end of the accelerated cooling. As a result, pro-eutectoid cementite structures form due to the temperature rise before pearlite structures form in a sufficient amount. For those reasons, the present invention stipulates that accelerated cooling should be applied until the temperature reaches a temperature not higher than 650° C.
No lower limit is particularly specified for the temperature at which accelerated cooling is finished but, for suppressing the formation of pro-eutectoid cementite structures and preventing the formation of martensite structures which form, more at segregated portions, in a web portion, 500° C. is the lower limit temperature substantially.
Next, the reasons are explained in detail for limiting the time period from the end of hot rolling to the beginning of heating at the web portion of a rail and the temperature range of the heating in their respective ranges.
First, explanations are given regarding the time period from the end of hot rolling to the beginning of heating at the web portion of a rail. When the time period from the end of hot rolling to the beginning of heating at the web portion of a rail exceeds 100 sec., with the chemical composition according to the present invention, pro-eutectoid cementite structures form in the web portion of the rail before the heating, and, even though the web portion is heated, the pro-eutectoid cementite structures remain the subsequent heat treatment and the fatigue strength and toughness of the rail deteriorate. For those reasons, the time period till the beginning of heating is limited to not longer than 100 sec.
No lower limit is particularly specified for the time period from the end of hot rolling to the beginning of heating at the web portion of a rail but, for mitigating the temperature unevenness occurring during rolling and carrying out the heating accurately, it is desirable to begin the heating after the lapse of not less than 5 sec. from the end of hot rolling.
Next, explanations are given regarding the temperature range of heating at the web portion of a rail. When the temperature rise of heating is less than 20° C., pro-eutectoid cementite structures form in the web portion of a rail before the subsequent accelerated cooling and the fatigue strength and toughness of the web portion of the rail deteriorate. When the temperature rise of heating exceeds 100° C., on the other hand, pearlite structures coarsen after heat treatment and the toughness of the web portion of a rail deteriorates. For those reasons, the temperature rise of heating at the web portion of a rail is limited in the range from 20° C. to 100° C.
Next, the reasons are explained for specifying the conditions of the heat treatment of the base toe portions of a rail. First, explanations are given regarding the time period from the end of hot rolling to the beginning of accelerated cooling at the base toe portions of a rail. When the time period from the end of hot rolling to the beginning of accelerated cooling at the base toe portions of a rail exceeds 60 sec., with the chemical composition according to the present invention, pro-eutectoid cementite structures form in the base toe portions of the rail before the accelerated cooling and the fatigue strength and toughness of the rail deteriorate. For those reasons, the time period till the beginning of accelerated cooling is limited to not longer than 60 sec.
No lower limit is particularly limited for the time period from the end of hot rolling to the beginning of accelerated cooling at the base toe portions of a rail but, to make uniform the size of austenite grains in the base toe portions of a rail and mitigating the temperature unevenness occurring during rolling, it is desirable to begin accelerated cooling after the lapse of not shorter than 5 sec. from the end of hot rolling.
Next, explanations are given regarding the range of the cooling rate of accelerated cooling at the base toe portions of a rail. When a cooling rate is below 5° C./sec., with the chemical composition according to the present invention, it becomes difficult to suppress the formation of pro-eutectoid cementite structures in the base toe portions of a rail. When a cooling rate exceeds 20° C./sec., on the other hand, with the chemical composition according to the present invention, martensite structures form in the base toe portions of a rail and the toughness of the base toe portions of the rail significantly deteriorates. For those reasons, an accelerated cooling rate at the base toe portions of a rail is limited in the range from 5 to 20° C./sec.
Note that the accelerated cooling rate at the base toe portions of a rail mentioned above is not a cooling rate during cooling but an average cooling rate from the beginning to the end of accelerated cooling. Therefore, as far as the average cooling rate from the beginning to the end of accelerated cooling is within the range specified above, it is possible to suppress the formation of pro-eutectoid cementite structures.
Next, explanations are given regarding the temperature range of accelerated cooling at the base toe portions of a rail. When accelerated cooling is finished at a temperature above 650° C., an excessive thermal recuperation takes place from the inside of a rail after the end of accelerated cooling. As a result, pro-eutectoid cementite structures form due to the temperature rise before pearlite structures form in a sufficient amount. For those reasons, the present invention stipulates that accelerated cooling should be applied until the temperature reaches a temperature not higher than 650° C.
Next, the reasons are explained in detail for limiting the time period from the end of hot rolling to the beginning of heating at the base toe portions of a rail and the temperature range of the heating in their respective ranges.
First, explanations are given regarding the time period from the end of hot rolling to the beginning of heating at the base toe portions of a rail. When the time period from the end of hot rolling to the beginning of heating at the base toe portions of a rail exceeds 60 sec., with the chemical composition according to the present invention, pro-eutectoid cementite structures form in the base toe portions of the rail before the heating, and, even though the base toe portions are heated thereafter, the pro-eutectoid cementite structures remain the subsequent heat treatment and the fatigue strength and toughness of the rail deteriorate. For those reasons, the time period till the beginning of heating is limited to not longer than 60 sec.
No lower limit is particularly limited for the time period from the end of hot rolling to the beginning of heating at the base toe portions of a rail but, for mitigating the temperature unevenness occurring during rolling and carrying out the heating accurately, it is desirable to begin the heating after the lapse of not less than 5 sec. from the end of hot rolling.
Next, explanations are given regarding the temperature range of heating at the base toe portions of a rail. When the temperature rise of heating is less than 50° C., pro-eutectoid cementite structures form in the base toe portions of a rail before the subsequent accelerated cooling and the fatigue strength and toughness of the base toe portions of the rail deteriorate. When the temperature rise of heating exceeds 100° C., on the other hand, pearlite structures coarsen after the heat treatment and the toughness of the base toe portions of a rail deteriorates. For those reasons, the temperature rise of heating at the base toe portions of a rail is limited in the range from 50° C. to 100° C.
With regard to the conditions of a railhead portion in the event of applying the above heat treatment, it is desirable to set the time period from the end of hot rolling to the heat treatment at not longer than 200 sec. and the area reduction ratio at the final pass of the finish hot rolling at 6% or more, or it is more desirable to apply continuous finish rolling of two or more passes with a time period of not longer than 10 sec. between passes at an area reduction ratio of 1 to 30% per pass.
(11) Reasons for Limiting the Length of a Rail after Hot Rolling
Here, the reasons are explained in detail for limiting the length of a rail after hot rolling.
When the length of a rail after hot rolling exceeds 200 m, the temperature difference between the ends and the middle portion and moreover between the two ends of the rail after the rolling becomes so large that it becomes difficult to properly control the temperature and the cooling rate over the whole rail length even though the above rail production method is employed, and the material quality of the rail in the longitudinal direction becomes uneven. When the length of a rail after hot rolling is less than 100 m, on the other hand, rolling efficiency lowers and the production cost of the rail increases. For these reasons, the length of a rail after hot rolling is limited in the range from 100 to 200 m.
Note that, in order to obtain a product rail length in the range from 100 to 200 m, it is desirable to secure a rolling length of the product rail length plus crop allowances.
(12) Reasons for Limiting Rolling Conditions at Hot Rolling
Here, the reasons are explained in detail for limiting rolling conditions at hot rolling.
When a temperature at the end of hot rolling exceeds 1,000° C., with the chemical composition according to the present invention, pearlite structures in a railhead portion are not made fine and ductility is not improved sufficiently. When a temperature at the end of hot rolling is below 850° C., on the other hand, it becomes difficult to control the shape of a rail and, as a result, to produce a rail satisfying a required product shape. In addition, pro-eutectoid cementite structures form immediately after the rolling owing to the low temperature and the fatigue strength and toughness of a rail deteriorate. For those reasons, a temperature at the end of hot rolling is limited in the range from 850° C. to 1,000° C.
When an area reduction ratio at the final pass of hot rolling is below 6%, it becomes impossible to make a austenite grain size fine after the rolling of a rail and, as a consequence, a pearlite block size increases and it is impossible to secure a high ductility at the railhead portion. For those reasons, an area reduction ratio at the final rolling pass is defined as 6% or more.
In addition to the above control of a rolling temperature and an area reduction ratio, for the purpose of improving ductility at a railhead portion, 2 or more consecutive rolling passes are applied at final rolling and, moreover, an area reduction ratio per pass and a time period between the passes at final rolling are controlled.
Next, the reasons are explained in detail for limiting an area reduction ratio per pass and a time period between the passes at final rolling.
When an area reduction ratio per pass at final rolling is less than 1%, austenite grains are not made fine at all, a pearlite block size is not reduced as a consequence, and thus ductility at a railhead portion is not improved. For those reasons, an area reduction ratio per pass at final rolling is limited to 1% or more. When an area reduction ratio per pass at final rolling exceeds 30%, on the other hand, it becomes impossible to control the shape of a rail and thus it becomes difficult to produce a rail satisfying a required product shape. For those reasons, an area reduction ratio per pass at final rolling is limited in the range from 1 to 30%.
When a time period between passes at final rolling exceeds 10 sec., austenite grains grow after the rolling, a pearlite block size is not reduced as a consequence, and thus ductility at a railhead portion is not improved. For those reasons, a time period between passes at final rolling is limited to not longer than 10 sec. No lower limit is particularly specified for a time period between passes but, for suppressing grain growth, making austenite grains fine through continuous recrystallization, and making a pearlite block size small as a result, it is desirable to make the time period as short as possible.
Here, the portions of a rail are explained.
Note that it is desirable to make the cooling rates at the above four measurement points as equal as possible in order to make uniform the hardness and the structures in a rail section.
A temperature at the rolling of a rail is represented by the temperature measured immediately after rolling at the point in the center of the rail width on the surface of the head portion 1 shown in
The present inventors also examined, in a steel rail of pearlite structures having a high carbon content, the relationship between the cooling rate capable of preventing pro-eutectoid cementite structures from forming at the head inner portion (critical cooling rate of pro-eutectoid cementite structure formation) and the chemical composition of the steel rail.
As a result of heat treatment tests using high-carbon steel specimens simulating the shape of a railhead portion, it has been clarified that: there is a relationship between the chemical composition (C, Si, Mn and Cr) of a steel rail and the critical cooling rate of pro-eutectoid cementite structure formation; and C, which is an element that accelerates the formation of cementite, has a positive correlation and Si, Mn and Cr, which are elements that increase hardenability, have negative correlations.
On the basis of the above finding, the present inventors tried to determine, in steel rails containing over 0.85 mass % C, wherein the formation of pro-eutectoid cementite structures is conspicuous, the relationship between the chemical composition (C, Si, Mn and Cr) of the steel rails and the critical cooling rates of pro-eutectoid cementite structure formation, by using multiple correlation analysis. As a result, it has been found that: the value corresponding to the critical cooling rate of pro-eutectoid cementite structure formation at the head inner portion of a steel rail is obtained by calculating the value of CCR defined by the equation (4) representing the contribution of chemical composition (mass %) in the steel rail; and further it is possible to prevent pro-eutectoid cementite structures from forming at the railhead inner portion by controlling the cooling rate at the railhead inner portion (ICR, ° C./sec.) to not less than the value of CCR in the heat treatment of a steel rail:
CCR=0.6+10×([% C]−0.9)−5×([% C]−0.9)×[% Si]−0.17[% Mn]−0.13[% Cr] (4).
Next, the present inventors studied a method for controlling a cooling rate at a head inner portion (ICR, ° C./sec.) in the heat treatment of a steel rail.
In view of the fact that the entire surface of a railhead portion is cooled in the event of cooling the railhead portion in a heat treatment, the present inventors carried out heat treatment tests using high-carbon steel specimens simulating the shape of a railhead portion and tried to find out the relationship between cooling rates at different positions on the surface of a railhead portion and a cooling rate at a railhead inner portion. As a result, it has been confirmed that: a cooling rate at a railhead inner portion correlates with a cooling rate at the surface of a railhead top portion (TH, ° C./sec.), the average of cooling rates at the surfaces of the right and left sides of a railhead portion (TS, ° C./sec.) and the average of cooling rates at the surfaces of the lower chin portions (TJ, ° C./sec.) that are located at the boundaries between the head and web portions on the right and left sides; and the cooling rate at the railhead inner portion can be evaluated by using the value of TCR defined by the equation (5) representing the contribution to the cooling rate at the railhead inner portion:
TCR=0.05TH(° C./sec.)+0.10TS(° C./sec.)+0.50TJ(° C./sec.) (5).
Note that each of the cooling rates at head side portions and lower chin portions (TS and TJ, ° C./sec.) is the average value of the cooling rates at the respective positions on the right and left sides of a rail.
Further, the present inventors experimentally investigated the relationship of the value of TCR with the formation of pro-eutectoid cementite structures in a railhead inner portion and structures in the surface layer of a railhead portion. As a result, it has been clarified that: the formation of pro-eutectoid cementite structures in a railhead inner portion correlates with the value of TCR; and, when the value of TCR is twice or more the value of CCR calculated from the chemical composition of a steel rail, pro-eutectoid cementite structures do not form in the railhead inner portion.
It has further been clarified that, in relation to the microstructures in the surface layer of a railhead portion, when the value of TCR is four times or more the value of CCR calculated from the chemical composition of a steel rail, the cooling is excessive, bainite and martensite structures detrimental to wear resistance form in the surface layer of the railhead portion, and the service life of the steel rail shortens.
That is, the present inventors have found out that, in the heat treatment of a railhead portion, it is possible to secure an appropriate cooling rate at the railhead inner portion (ICR, ° C./sec.), prevent the formation of pro-eutectoid cementite structures there, and additionally stabilize pearlite structures in the surface layer of the railhead portion by controlling the value of TCR so as to satisfy the expression 4CCR≧TCR≧2CCR.
To sum up, the present inventors have found that, in a steel rail having a high carbon content: it is possible to prevent the formation of pro-eutectoid cementite structures in the head inner portion of the steel rail by controlling the cooling rate at the head inner portion (ICR) so as to be not less than the value of CCR calculated from the chemical composition of the steel rail; and moreover it is necessary to control the value of TCR calculated from the cooling rates at the different positions on the surface of the head portion within the range regulated by the value of CCR for securing an appropriate cooling rate at the head inner portion (ICR) and stabilizing pearlite structures in the surface layer of the head portion.
Accordingly, the present invention makes it possible to, in the heat treatment of a high-carbon steel rail used in a heavy load railway: stabilize pearlite structures in the surface layer of the head portion; at the same time, prevent the formation of pro-eutectoid cementite structures, which are likely to form at the head inner portion and serve as the origin of fatigue damage; and, as a consequence, secure a good wear resistance and improve resistance to internal fatigue damage.
(13) Reasons for Regulating the Heat Treatment Method for Preventing the Formation of Pro-Eutectoid Cementite Structures in a Railhead Inner Portion
1) Reasons for Defining the Equation for Calculating the Value of CCR
The reasons are explained for defining the equation for calculating the value of CCR as described above.
The equation for calculating the value of CCR has been derived from the procedures of: firstly measuring the critical cooling rate of pro-eutectoid cementite structure formation through the tests simulating the heat treatment of a railhead portion; and then clarifying the relationship between the critical cooling rate of pro-eutectoid cementite structure formation and the chemical composition (C, Si, Mn and Cr) of a steel rail by using multiple correlation analysis. The resulting correlation equation (4) is shown below. As stated above, the equation (4) is an experimental regression equation, and it is possible to prevent the formation of pro-eutectoid cementite structures by cooling a railhead inner portion at a cooling rate not lower than the value calculated from the equation (4):
CCR=0.6+10×([% C]−0.9)−5×([% C]−0.9)×[% Si]−0.17[% Mn]−0.13[% Cr] (4).
2) Reasons for Limiting a Position and a Temperature Range Wherein a Cooling Rate at a Railhead Inner Portion is Regulated
The reasons are explained for determining a position where a cooling rate at a railhead inner portion is regulated to be a position 30 mm in depth from a head top surface.
A cooling rate at a railhead portion tends to decrease from the surface toward the inside thereof. Therefore, in order to prevent pro-eutectoid cementite structures from forming at the regions of the railhead portion where the cooling rate is lower, it is necessary to secure an adequate cooling rate at the railhead inner portion. As a result of experimentally measuring the cooling rates at different positions in a railhead inner portion, it has been confirmed that: the cooling rate at the position 30 mm in depth from a head top surface is the lowest; and, when an adequate cooling rate is secured at this position, pro-eutectoid cementite structures are prevented from forming at the railhead inner portion. From the results, the position where a cooling rate at a railhead inner portion is regulated is determined to be a position 30 mm in depth from a head top surface.
Next, the reasons are explained for defining a temperature range in which a cooling rate at a railhead inner portion is regulated.
It has been experimentally confirmed that, in a steel rail having the chemical composition as specified above, the temperature at which pro-eutectoid cementite structures form is in the range from 750° C. to 650° C. Therefore, in order to prevent the formation of pro-eutectoid cementite structures, it is necessary to control a cooling rate at a railhead inner portion to at least a certain value or more in the above temperature range. For those reasons, a temperature range in which a cooling rate at the position 30 mm in depth from the head top surface of a steel rail is regulated is determined to be from 750° C. to 650° C.
3) Reasons for Defining the Equation for Calculating the Value of TCR and Limiting the Range of the Value
The reasons are explained for defining the equation for calculating the value of TCR.
The equation for calculating the value of TCR has been derived from the procedures of: firstly measuring a cooling rate at a railhead top portion (TH, ° C./sec.), a cooling rate at railhead side portions (TS, ° C./sec.), a cooling rate at lower chin portions (TJ, ° C./sec.), and moreover a cooling rate at a railhead inner portion (ICR, ° C./sec.) through the tests simulating the heat treatment of a railhead portion; and then formulating the cooling rates at the respective railhead surface portions according to their contributions to the cooling rate at the railhead inner portion (ICR, ° C./sec.). The resulting equation (5) is shown below. As stated above, the equation (5) is an empirical equation and, as far as a value calculated from the equation (5) is not less than a certain value, it is possible to secure an adequate cooling rate at a railhead inner portion and prevent the formation of pro-eutectoid cementite structures:
TCR=0.05TH(° C./sec.)+0.10TS(° C./sec.)+0.50TJ(° C./sec.) (5).
Note that each of the cooling rates at head side portions and lower chin portions (TS and TJ, ° C./sec.) is the average value of the cooling rates at the respective positions on the right and left sides of a rail.
Next, the reasons are explained for regulating the value of TCR so as to satisfy the expression 4CCR≧TCR≧2CCR.
When the value of TCR is smaller than 2CCR, a cooling rate at a railhead inner portion (ICR, ° C./sec.) decreases, pro-eutectoid cementite structures form in the railhead inner portion, and internal fatigue damage is likely to occur. In addition, in that case, the hardness at the surface of a railhead portion deteriorates and a good wear resistance of a rail cannot be secured. When the value of TCR exceeds 4CCR, on the other hand, cooling rates at the surface layer of a railhead portion increase drastically, bainite and martensite structures detrimental to wear resistance form in the surface layer of the railhead portion, and the service life of the steel rail shortens. For those reasons, the value of TCR is restricted in the range specified by the expression 4CCR≧TCR≧2CCR.
4) Reasons for Limiting Positions and a Temperature Range wherein Cooling Rates at the Surface of a Railhead Portion are Regulated
In the first place, the reasons are explained for determining positions where cooling rates at the surface of a railhead portion are regulated to be three kinds of portions; a head top portion, head side portions and lower chin portions.
A cooling rate at a railhead inner portion is significantly influenced by cooling conditions at the surface of a railhead portion. The present inventors experimentally examined the relationship between a cooling rate at a railhead inner portion and cooling rates at the surface of a railhead portion. As a result, it has been confirmed that: a cooling rate at a railhead inner portion is in good correlation with cooling rates at three kinds of surfaces, through which heat at a railhead portion is removed, of the top, the sides (right and left) and the lower chins (right and left) of the railhead portion; and a cooling rate at a rail head inner portion is adequately controlled by adjusting cooling rates at the surfaces. From the results, the positions where cooling rates at the surface of a railhead portion are regulated are determined to be the top, the sides and the lower chins of the railhead portion.
Next, the reasons are explained for defining a temperature range in which cooling rates at the three kinds of surfaces of a railhead portion are regulated.
It has been experimentally confirmed that, in a steel rail having the chemical composition as specified above, the temperature at which pro-eutectoid cementite structures form is in the range from 750° C. to 650° C. Therefore, in order to prevent the formation of pro-eutectoid cementite structures, it is necessary to control a cooling rate at a railhead inner portion to at least a certain value or more in the above temperature range. However, as the amount of heat removed at a railhead inner portion is smaller than that removed at the surface of a railhead portion at the time of the end of accelerated cooling, the temperature at the railhead inner portion is higher than that at the surface of the railhead portion. Accordingly, in order to secure an adequate cooling rate at a railhead inner portion in the temperature range down to 650° C., beyond which pro-eutectoid cementite structures form, it is necessary to regulate a temperature at the end of accelerated cooling to below 650° C. at the surface of the railhead portion. As a result of verifying experimentally the temperature at the end of accelerated cooling at the surface of a railhead portion, it has been confirmed that, when a cooling is continued until a surface temperature reaches 500° C., a temperature at the end of cooling at a railhead inner portion falls to below 650° C. From those results, a temperature range in which cooling rates at the three kinds of surfaces of a railhead portion (the top, the sides and the lower chins of a railhead portion) are regulated is determined to be from 750° C. to 500° C.
Here, the portions of a rail are explained.
Accelerated cooling rates and temperature ranges of accelerated cooling in the heat treatment of a rail are defined by the relevant representative values that are measured on the surfaces of, or in the regions up to 5 mm in depth from the surfaces of, as shown in
As a consequence, by controlling temperatures and cooling rates at the above portions, it is possible to stabilize pearlite structures in the surface layer of a head portion and control a cooling rate at a head inner portion 4, thus secure a good wear resistance at the surface of the head portion, prevent the formation of pro-eutectoid cementite structures at the head inner portion, and, in addition, enhance resistance to internal fatigue damage. With regard to accelerated cooling during the heat treatment of a railhead portion, it is possible to arbitrarily choose, as required, the application or otherwise of cooling and accelerated cooling rates in the case of the application at the five positions, namely a head top portion, head side portions (right and left) and lower chin portions (right and left), so that the value of TCR may satisfy the expression 4CCR≧TCR≧2CCR.
Note that it is desirable to make cooling rates on both the right and left sides of head side portions and lower chin portions equal in order to make hardness and metallographic structures uniform on both the sides of a railhead portion.
As explained above, in order to prevent the formation of pro-eutectoid cementite structures at a head inner portion and stabilize pearlite structures in the surface layer of a head portion in a steel rail of pearlite structures having a high carbon content, it is necessary to control a cooling rate at the head inner portion (ICR) so as to be not lower than the value of CCR that is determined by the chemical composition of the steel rail and corresponds to the critical cooling rate under which cementite structures form, and, at the same time, to control cooling rates at the aforementioned different positions on the surfaces of the railhead portion so that the value of TCR may fall within the specified range.
It is desirable that the metallographic structure of a steel rail produced through a heat treatment method according to the present invention is composed of pearlite structures almost over the entire body. In some choices of chemical composition and accelerated cooling conditions, pro-eutectoid ferrite structures, pro-eutectoid cementite structures and bainite structures may form in very small amounts in pearlite structures. However, as long as the amounts of these structures are very small, their presence in pearlite structures does not have a significant influence on the fatigue strength and the toughness of a rail. For this reason, the structure of the head portion of a steel rail produced through a heat treatment method according to the present invention may include pearlite structures in which small amounts of pro-eutectoid ferrite structures, pro-eutectoid cementite structures and bainite structures are mixed.
Table 1 shows, regarding each of the steel rails according to the present invention, chemical composition, hot rolling and heat treatment conditions, the microstructure of a head portion at a depth of 5 mm from the surface thereof, the number and the measurement position of pearlite blocks having grain sizes in the range from 1 to 15 μm, and the hardness of a head portion at a depth of 5 mm from the surface thereof. Table 1 also shows the amount of wear of the material at a head portion after 700,000 repetition cycles of Nishihara wear test are imposed under the condition of forced cooling as shown in
Table 2 shows, regarding each of the comparative steel rails, chemical composition, hot rolling and heat treatment conditions, the microstructure of a head portion at a depth of 5 mm from the surface thereof, the number and the measurement position of pearlite blocks having grain sizes in the range from 1 to 15 μl, and the hardness of a head portion at a depth of 5 mm from the surface thereof. Table 2 also shows the amount of wear of the material at a head portion after 700,000 repetition cycles of Nishihara wear test are imposed under the condition of forced cooling as shown in
Note that any of the steel rails listed in Tables 1 and 2 was produced under the conditions of a time period of 180 sec. from hot rolling to heat treatment and an area reduction ratio of 6% at the final pass of finish hot rolling.
The rails listed in the tables are as follows:
Steel Rails According to the Present Invention (12 rails), Symbols 1 to 12
The pearlitic steel rails excellent in wear resistance and ductility having chemical composition in the aforementioned ranges, characterized in that the number of the pearlite blocks having grain sizes in the range from 1 to 15 μm is 200 or more per 0.2 mm2 of observation field at least in a part of the region down to a depth of 10 mm from the surface of the corners and top of a head portion.
Comparative Steel Rails (10 Rails), Symbols 13 to 22
Symbols 13 to 16 (4 rails): the comparative steel rails, wherein the amounts of C, Si, Mn in alloying are outside the respective ranges according to the claims of the present invention.
Symbols 17 to 22 (6 rails): the comparative steel rails having the chemical composition in the aforementioned ranges, wherein the number of the pearlite blocks having grain sizes in the range from 1 to 15 μm is less than 200 per 0.2 mm2 of observation field at least in a part of the region down to a depth of 10 mm from the surface of the corners and top of a head portion.
Here, explanations are given regarding the drawings attached hereto.
Further,
The tests were carried out under the following conditions:
Wear Test of a Head Portion
As seen in Tables 1 and 2, in the cases of the steel rails according to the present invention in contrast to the cases of the comparative steel rails, pro-eutectoid cementite structures, pro-eutectoid ferrite structures, martensite structures and so on detrimental to the wear resistance and ductility of a rail did not form and the wear resistance and ductility were good as a result of controlling the addition amounts of C, Si and Mn within the respective prescribed ranges.
In addition, as seen in
In addition, as seen in
TABLE 1
Chemical composition
(mass %)
Cr/Mo/V/Nb/B/
Classification
Co/Cu/Ni/Ti/
of rail
Symbol
Steel
C
Si
Mn
Mg/Ca/Al/Zr
Hot rolling and heat treatment conditions
Invented
1
1
0.68
0.25
0.80
Ni: 0.15
Area reduction ratio of final rolling: 13%
rail
Rolling end temperature: 940° C.
Accelerated cooling rate: 5° C./sec
2
2
0.75
0.15
1.31
Cu: 0.15
Area reduction ratio of final rolling: 10%
Rolling end temperature: 950° C.
Accelerated cooling rate: 4° C./sec
3
3
0.80
0.30
0.98
Reheating temperature: 870° C.
Accelerated cooling rate: 7° C./sec
4
4
0.85
0.45
1.00
Mo: 0.02
Area reduction ratio of final rolling: 9%
Co: 0.21
Rolling end temperature: 940° C.
Accelerated cooling rate: 4° C./sec
5
5
0.87
0.52
1.15
Mg: 0.0021
Area reduction ratio of final rolling: 12%
Ca: 0.0012
Rolling end temperature: 930° C.
Accelerated cooling rate: 5° C./sec
6
6
0.91
0.25
0.60
V: 0.04
Area reduction ratio of final rolling: 9%
Rolling end temperature: 980° C.
Accelerated cooling rate: 5° C./sec
7
7
0.94
0.75
0.80
Cr: 0.45
Area reduction ratio of final rolling: 8%
Rolling end temperature: 960° C.
Accelerated cooling rate: 3° C./sec
8
8
1.01
0.81
1.05
B: 0.0012
Area reduction ratio of final rolling: 11%
Rolling end temperature: 960° C.
Accelerated cooling rate: 6° C./sec
9
9
1.04
0.41
0.75
Cr: 0.21
Area reduction ratio of final rolling: 10%
Rolling end temperature: 950° C.
Accelerated cooling rate: 5° C./sec
10
10
1.10
0.45
1.65
Zr: 0.0015
Area reduction ratio of final rolling: 15%
Nb: 0.018
Rolling end temperature: 935° C.
Accelerated cooling rate: 6° C./sec
11
11
1.20
1.21
0.65
Ti: 0.0130
Area reduction ratio of final rolling: 10%
Al: 0.0400
Rolling end temperature: 920° C.
Accelerated cooling rate: 8° C./sec
12
12
1.38
1.89
0.20
Al: 0.18
Reheating temperature: 900° C.
Accelerated cooling rate: 10° C./sec
Hardness of
Tensile
Microstructure
Number of
head
test
of head
pearlite blocks 1
portion
result of
portion
to 15 μm in grain
(5 mm in
Amount
head
(5 mm in
size
depth from
of wear
portion
depth
(per 0.2 mm2)
head
of head
Total
Classification
from head
Measurement
surface)
portion
elongation
of rail
Symbol
surface)
position
(Hv 10 kgf)
(g)
(%)
Invented
1
Pearlite
405
335
1.35
22.5
rail
5 mm in depth
from head surface
2
Pearlite
231
358
1.24
18.3
4 mm in depth
from head surface
3
Pearlite
765
395
1.15
20.5
8 mm in depth
from head surface
4
Pearlite
321
405
1.08
16.0
6 mm in depth
from head surface
5
Pearlite
380
415
0.88
15.8
3 mm in depth
from head surface
6
Pearlite
212
385
0.85
14.5
1 mm in depth
from head surface
7
Pearlite
248
389
0.75
12.9
3 mm in depth
from head surface
8
Pearlite
285
448
0.59
11.9
2 mm in depth
from head surface
9
Pearlite
265
422
0.62
10.9
3 mm in depth
from head surface
10
Pearlite
348
452
0.52
11.0
6 mm in depth
from head surface
11
Pearlite
325
478
0.36
10.0
7 mm in depth
from head surface
12
Pearlite
574
415
0.30
11.5
9 mm in depth
from head surface
Note:
Balance of chemical composition is Fe and unavoidable impurities.
TABLE 2
Microstructure
of head
Chemical composition
portion
(mass %)
(5 mm in
Cr/Mo/V/Nb/B/
depth
Classification
Co/Cu/Ni/Ti/
Hot rolling and heat
from head
of rail
Symbol
Steel
C
Si
Mn
Mg/Ca/Al/Zr
treatment conditions
surface)
Comparative
13
13
0.60
0.25
0.80
Ni: 0.12
Area reduction ratio of final
Pearlite + pro-
rail
rolling: 13%
eutectoid
Rolling end temperature: 940° C.
ferrite
Accelerated cooling rate: 3° C./sec
14
14
1.45
1.75
0.20
Al: 0.18
Area reduction ratio of final
Pearlite + pro-
rolling: 9%
eutectoid
Rolling end temperature: 970° C.
cementite
Accelerated cooling rate: 5° C./sec
15
15
0.87
2.15
1.16
Mg: 0.0015
Area reduction ratio of final
Pearlite
Ca: 0.0012
rolling: 12%
Rolling end temperature: 930° C.
Accelerated cooling rate: 5° C./sec
16
16
0.75
0.16
2.25
Cu: 0.16
Area reduction ratio of final
Pearlite
rolling: 10%
Rolling end temperature: 950° C.
Accelerated cooling rate: 4° C./sec
17
17
1.04
0.41
0.76
Cr: 0.21
Area reduction ratio of final
Pearlite
rolling: 5%
Rolling end temperature: 960° C.
Accelerated cooling rate: 5° C./sec
18
18
1.01
0.81
1.02
B: 0.0015
Area reduction ratio of final
Pearlite
rolling: 10%
Rolling end temperature: 1000° C.
Accelerated cooling rate: 5° C./sec
19
19
0.91
0.26
0.61
V: 0.03
Area reduction ratio of final
pearlite
rolling: 5%
Rolling end temperature: 990° C.
Accelerated cooling rate: 5° C./sec
20
20
0.94
0.71
0.75
Cr: 0.44
Area reduction ratio of final
Pearlite
rolling: 5%
Rolling end temperature: 1020° C.
Accelerated cooling rate: 3° C./sec
21
21
1.20
1.15
0.60
Ti: 0.0125
Area reduction ratio of final
Pearlite
Al: 0.0300
rolling: 5%
Rolling end temperature: 920° C.
Accelerated cooling rate: 8° C./sec
22
22
1.38
1.75
0.25
Al: 0.15
Reheating temperature: 1050° C.
Pearlite
Accelerated cooling rate: 6° C./sec
Hardness of
Number of
head
pearlite blocks 1
portion
to 15 μm in grain
(5 mm in
Amount of
Tensile test
size
depth from
wear of
result of head
(per 0.2 mm2)
head
head
portion
Classification
Measurement
surface)
portion
Total elongation
of rail
Symbol
position
(Hv 10 kgf)
(g)
(%)
Comparative
13
380
315
Low carbon
22.0
rail
5 mm in depth
content,
from head surface
large wear
1.72
14
205
375
0.34
Pro-eutectoid
3 mm in depth
cementite formed
from head surface
→ low ductility
8.9
15
370
435
0.90
Excessive Si,
3 mm in depth
structure
from head surface
embrittled, low
ductility
12.0
16
240
528
Martensite
Martensite
4 mm in depth
formed,
formed, low
from head surface
large wear
ductility
2.45
5.2
17
155
432
0.60
Fine pearlite
3 mm in depth
blocks decreased
from head surface
→ low ductility
8.6
18
102
452
0.57
Fine pearlite
2 mm in depth
blocks decreased
from head surface
→ low ductility
8.8
19
95
394
0.82
Fine pearlite
1 mm in depth
blocks decreased
from head surface
→ low ductility
10.0
20
56
405
0.71
Fine pearlite
3 mm in depth
blocks decreased
from head surface
→ low ductility
9.2
21
175
480
0.34
Fine pearlite
7 mm in depth
blocks decreased
from head surface
→ low ductility
7.8
22
56
425
0.34
Fine pearlite
9 mm in depth
blocks decreased
from head surface
→ low ductility
6.5
Note:
Balance of chemical composition is Fe and unavoidable impurities.
Table 3 shows, regarding each of the steel rails according to the present invention, chemical composition, hot rolling and heat treatment conditions, the microstructure of a head portion at a depth of 5 mm from the surface thereof, the number and the measurement position of pearlite blocks having grain sizes in the range from 1 to 15 μm, and the hardness of a head portion at a depth of 5 mm from the surface thereof. Table 3 also shows the amount of wear of the material at a head portion after 700,000 repetition cycles of Nishihara wear test are imposed under the condition of forced cooling as shown in
Table 4 shows, regarding each of the comparative steel rails, chemical composition, hot rolling and heat treatment conditions, the microstructure of a head portion at a depth of 5 mm from the surface thereof, the number and the measurement position of pearlite blocks having grain sizes in the range from 1 to 15 μm, and the hardness of a head portion at a depth of 5 mm from the surface thereof. Table 4 also shows the amount of wear of the material at a head portion after 700,000 repetition cycles of Nishihara wear test are imposed under the condition of forced cooling as shown in
Note that any of the steel rails listed in Tables 3 and 4 was produced under the condition of an area reduction ratio of 6% at the final pass of finish hot rolling.
The rails listed in the tables are as follows:
Steel Rails According to the Present Invention (16 Rails), Symbols 23 to 38
The pearlitic steel rails excellent in wear resistance and ductility having chemical composition in the aforementioned ranges, characterized in that the number of the pearlite blocks having grain sizes in the range from 1 to 15 μm is 200 or more per 0.2 mm2 of observation field at least in a part of the region down to a depth of 10 mm from the surface of the corners and top of a head portion.
Comparative Steel Rails (16 Rails), Symbols 39 to 54
Symbols 39 to 42 (4 rails): the comparative steel rails, wherein the amounts of C, Si, Mn in alloying were outside the respective ranges according to the claims of the present invention.
Symbol 43 (1 rail): the comparative steel rail having the rail length outside the range according to the claims of the present invention.
Symbols 44 and 47 (2 rails): the comparative steel rails, wherein a time period from the end of rolling to the beginning of accelerated cooling is outside the range according to the claims of the present invention.
Symbols 45, 46 and 48 (3 rails): the comparative steel rails, wherein an accelerated cooling rate at a head portion is outside the range according to the claims of the present invention.
Symbols 49 to 54 (6 rails): the comparative steel rails having the chemical composition in the aforementioned ranges, wherein the number of the pearlite blocks having grain sizes in the range from 1 to 15 μm is less than 200 per 0.2 mm2 of observation field at least in a part of the region down to a depth of 10 mm from the surface of the corners and top of a head portion.
The tests were carried out under the same conditions as in Example 1.
As seen in Tables 3 and 4, in the cases of the steel rails according to the present invention in contrast to the cases of the comparative steel rails, pro-eutectoid cementite structures, pro-eutectoid ferrite structures, martensite structures and so on detrimental to the wear resistance and ductility of a rail did not form and the wear resistance and ductility were good as a result of controlling the amounts of C, Si, Mn in alloying, the rail lengths at the rolling and the time periods from the end of rolling to the beginning of accelerated cooling within the respective prescribed ranges.
In addition, as seen in Tables 3 and 4, in the cases of the steel rails according to the present invention in contrast to the cases of the comparative steel rails, the ductility of the railhead portions improved as a result of controlling the numbers of the pearlite blocks having grain sizes in the range from 1 to 15 μm. Thus, it was possible to prevent the fractures such as breakage of a rail in cold regions.
TABLE 3
Time from end
Accelerated
of hot
cooling
Chemical composition
Rail
rolling to
conditions of
(mass %)
length
beginning of
head portion
Cr/Mo/V/Nb/B/
at hot
accelerated
Top: Cooling rate
Classification
Co/Cu/Ni/Ti/
rolling
cooling
Bottom: Cooling
of rail
Symbol
Steel
C
Si
Mn
Mg/Ca/Al/Zr/N
(m)
(sec)
end temperature
Invented
23
23
0.65
—
—
—
198
198
9° C./sec
rail
530° C.
24
24
0.68
0.25
0.80
Ni: 0.15
189
185
5° C./sec
510° C.
25
25
0.75
0.15
1.31
Cu: 0.15
165
170
4° C./sec
545° C.
26
26
0.80
0.30
0.98
—
175
185
7° C./sec
505° C.
27
27
0.85
0.45
1.00
Mo: 0.02
150
180
4° C./sec
Co: 0.21
489° C.
28
28
0.87
0.52
1.15
Mg: 0.0021
178
178
5° C./sec
Ca: 0.0012
475° C.
29
29
0.91
0.25
0.60
V: 0.02
155
158
6° C./sec
N: 0.0080
515° C.
30
30
0.91
0.25
0.60
V: 0.04
155
156
5° C./sec
500° C.
31
31
0.94
0.75
0.80
Cr: 0.45
165
156
3° C./sec
520° C.
32
32
1.01
—
—
—
165
135
12° C./sec
450° C.
33
33
1.01
0.40
1.05
Cr: 0.25
165
155
7° C./sec
450° C.
34
34
1.04
0.41
0.75
Cr: 0.21
150
115
10° C./sec
485° C.
35
35
1.10
0.45
1.65
Zr: 0.0015
135
115
6° C./sec
Nb: 0.018
485° C.
36
36
1.20
1.21
0.65
Ti: 0.0130
120
58
12° C./sec
Al: 0.0400
465° C.
37
37
1.38
1.89
0.20
Al: 0.18
110
25
18° C./sec
495° C.
38
38
1.38
0.15
0.20
B: 0.012
100
15
25° C./sec
485° C.
Hardness of
Tensile
Microstructure
Number of
head
test
of head
pearlite blocks 1
portion
result of
portion
to 15 μm in grain
(5 mm in
Amount of
head
(5 mm in
size
depth from
wear of
portion
depth
(per 0.2 mm2)
head
head
Total
Classification
from head
Measurement
surface)
portion
elongation
of rail
Symbol
surface)
position
(Hv 10 kgf)
(g)
(%)
Invented
23
Pearlite
223
305
1.45
22.5
rail
3 mm in depth
from head surface
24
Pearlite
445
335
1.35
23.5
5 mm in depth
from head surface
25
Pearlite
231
358
1.24
18.6
4 mm in depth
from head surface
26
Pearlite
285
395
1.15
14.0
8 mm in depth
from head surface
27
Pearlite
351
405
1.08
16.5
6 mm in depth
from head surface
28
Pearlite
405
415
0.91
16.2
3 mm in depth
from head surface
29
Pearlite
325
405
0.83
15.0
1 mm in depth
from head surface
30
Pearlite
242
385
0.85
14.8
1 mm in depth
from head surface
31
Pearlite
268
389
0.75
13.0
3 mm in depth
from head surface
32
Pearlite
225
398
0.65
10.8
2 mm in depth
from head surface
33
Pearlite
305
448
0.60
11.8
2 mm in depth
from head surface
34
Pearlite
285
432
0.60
12.0
3 mm in depth
from head surface
35
Pearlite
376
462
0.50
10.5
3 mm in depth
from head surface
36
Pearlite
345
488
0.38
10.2
2 mm in depth
from head surface
37
Pearlite
407
489
0.31
10.2
3 mm in depth
from head surface
38
Pearlite
305
465
0.35
10.0
3 mm in depth
from head surface
Note:
Balance of chemical composition is Fe and unavoidable impurities.
TABLE 4
Accelerated
cooling
Time from
conditions
end of hot
of head
Chemical composition
rolling to
portion
Microstructure
(mass %)
Rail
beginning
Top: Cooling
of
Cr/Mo/V/Nb/
length
of
rate
head portion
Classification
B/Co/Cu/Ni/
at hot
accelerated
Bottom:
(5 mm in
of
Ti/Mg/Ca/Al/
rolling
cooling
Cooling end
depth from
rail
Symbol
Steel
C
Si
Mn
Zr/N
(m)
(sec)
temperature
head surface)
Comparative
39
39
0.60
0.25
0.80
Ni: 0.12
150
198
3° C./sec
Pearlite + pro-
rail
550° C.
eutectoid
ferrite
40
40
1.45
1.75
0.20
Al: 0.18
105
100
5° C./sec
Peerlite + pro-
520° C.
eutectoid
cementite
41
41
0.87
2.15
1.16
Mg: 0.0015
155
160
5° C./sec
Pearlite
Ca: 0.0012
480° C.
42
42
0.75
0.16
2.25
Cu: 0.16
165
180
4° C./sec
Pearlite + martensite
480° C.
43
34
1.04
0.41
0.75
Cr: 0.21
250
115
10° C./sec
Pearlite + pro-
(Excessive
485° C.
eutectiod
rail
cementite
length)
44
36
1.20
1.21
0.65
Ti: 0.0130
120
265
12° C./sec
Pearlite + trace
Al: 0.0400
465° C.
Pro-eutectoid
cementite at
rail ends
45
35
1.10
0.45
1.65
Zr: 0.0015
110
115
0.5° C./sec
Pearlite + trace
Nb: 0.018
485° C.
pro-eutectoid
cementite
46
30
0.91
0.25
0.60
V: 0.04
155
156
35° C./sec
Pearlite + martensite
500° C.
Number of
Hardness of
pearlite
head
blocks 1 to
portion
15 μm in
(5 mm in
Tensile test
grain size
depth from
Amount of wear
result of head
(per 0.2 mm2)
head
of head
portion
Classification
Measurement
surface)
portion
Total elongation
of rail
Symbol
position
(Hv 10 kgf)
(g)
(%)
Comparative
39
250
315
Lowest carbon
22.0
rail
2 mm in depth
content, large
from head
wear
surface
1.72
40
205
375
0.34
Pro-eutectoid
3 mm in depth
cementite formed
from head
→ low ductility
surface
8.2
41
320
435
0.90
Excessive Si,
3 mm in depth
structure
from head
embrittled, low
surface
ductility
9.0
42
222
528
Martensite
Martensite formed,
4 mm in depth
formed, large
low ductility
from head
wear
5.2
surface
2.45
43
225
402
Pro-eutectoid
Pro-eutectoid
3 mm in depth
cementite
martensite formed,
from head
formed, large
low ductility
surface
wear
7.8
1.85
44
215
478
Pro-eutectoid
Pro-eutectoid
2 mm in depth
cementite
martensite formed,
from head
formed, large
low ductility
surface
wear
6.9
1.80
45
256
389
0.98
Trace pro-
3 mm in depth
eutectoid
from head
martensite formed,
surface
low ductility
7.2
46
286
548
Martensite
Martensite formed,
1 mm in depth
formed, large
low ductility
from head
wear
5.0
surface
2.25
Note:
Balance of chemical composition is Fe and unavoidable impurities.
TABLE 5
Accelerated
cooling
Time from
conditions
Chemical composition
end of hot
of head
(mass %)
rolling to
portion
Microstructure
Cr/Mo/V/
Rail
beginning
Top: Cooling
of
Nb/B/Co/
length at
of
rate
head portion
Classification
Cu/Ni/Ti/
hot
accelerated
Bottom:
(5 mm in
of
Mg/Ca/Al/
rolling
Cooling
Cooling end
depth from
rail
Symbol
Steel
C
Si
Mn
Zr/N
(m)
(sec)
temperature
head surface)
Comparative
47
23
0.65
—
—
—
198
300
9° C./sec
Pearlite
rail
530° C.
48
31
0.94
0.75
0.80
Cr: 0.45
165
156
0.5° C./sec
Pearlite
520° C.
49
29
0.91
0.25
0.60
V: 0.02
155
215
6° C./sec
Pearlite
N: 0.0080
515° C.
50
32
1.01
—
—
—
165
205
12° C./sec
Pearlite
450° C.
51
33
1.01
0.40
1.05
Cr: 0.25
165
235
7° C./sec
Pearlite
450° C.
52
35
1.10
0.45
1.65
Zr: 0.0015
135
225
8° C./sec
Pearlite
Nb: 0.018
485° C.
53
36
1.20
1.21
0.65
Ti: 0.0130
120
221
12° C./sec
Pearlite
Al: 0.0400
465° C.
54
37
1.38
1.89
0.20
Al: 0.18
110
201
18° C./sec
Pearlite
495° C.
Number of
Hardness of
pearlite
head
blocks 1 to
portion
15 μm in
(5 mm in
Tensile test
grain size
depth from
Amount of wear
result of head
Classification
(per 0.2 mm2)
head
of head
portion
of
Measurement
surface)
portion
Total elongation
rail
Symbol
position
(Hv 10 kgf)
(g)
(%)
Comparative
47
152
302
1.46
Pearlite block
rail
3 mm in depth
coarsened → low
from head
ductility
surface
18.5
48
150
280
1.25
Pearlite block
3 mm in depth
Softened,
coarsened → low
from head
pearlite
ductility
surface
coarsened
10.5
49
235
405
0.83
Fine pearlite
1 mm in depth
blocks decreased
from head
→ low ductility
surface
13.5
50
205
398
0.66
Fine pearlite
2 mm in depth
blocks decreased
from head
→ low ductility
surface
10.0
51
210
448
0.60
Fine pearlite
2 mm in depth
blocks decreased
from head
→ low ductility
surface
10.6
52
234
462
0.51
Fine pearlite
3 mm in depth
blocks decreased
from head
→ low ductility
surface
9.8
53
215
480
0.39
Fine pearlite
2 mm in depth
blocks decreased
from head
→ low ductility
surface
9.5
54
251
480
0.34
Fine pearlite
3 mm in depth
blocks decreased
from head
→ low ductility
surface
9.2
Note:
Balance of chemical composition is Fe and unavoidable impurities.
The same tests as in Examples 1 and 2 were carried out using the steel rails of Example 2 shown in Table 3 and changing the time period from the end of rolling to the beginning of accelerated cooling and the hot rolling conditions as shown in Table 6.
As is clear from Table 6, total elongation was further improved in the cases where the time periods from the end of rolling to the beginning of accelerated cooling were not longer than 200 sec., 2 or more passes of the finish hot rolling were applied, and the times between rolling passes were not longer than 10 sec.
TABLE 6
Time from
end of hot
rolling to
Rail
beginning
Hot rolling conditions
length
of
3
Time
2
Time
Time
Rolling
Classification
at hot
accelerated
passes
between
passes
between
1 pass
between
end
of
rolling
cooling
to
passes
to
passes
to
passes
Final
temperature
rail
Symbol
Steel
(m)
(sec)
final %
sec
final %
sec
final %
sec
pass %
° C.
Invented
55
23
198
198
—
6
980
rail
56
29
155
158
—
8
980
57
29
155
158
—
9
870
58
29
155
158
—
20
6
2
1
9
980
59
31
165
156
—
8
960
60
32
165
135
8
8
8
3
10
980
61
33
165
155
—
7
950
62
33
165
155
—
20
7
2
1
7
950
63
33
165
155
10
1
8
1
8
1
7
950
Accelerated
cooling
conditions
Number of
Hardness of
Tensile
of head
Microstructure
pearlite
head
test
portion
of head
blocks 1 to
portion
result of
Top: Cooling
portion
15 μm in
(5 mm in
Amount
head
rate
(5 mm in
grain size
depth from
of wear
portion
Classification
Bottom:
depth
(per 0.2 mm2)
head
of head
Total
of
Cooling end
from head
Measurement
surface)
portion
elongation
rail
Symbol
temperature
surface)
position
(Hv 10 kgf)
(g)
(%)
Invented
55
9° C./sec
Pearlite
253
305
1.45
24.5
rail
530° C.
3 mm in
depth from
head surface
56
6° C./sec
Pearlite
355
385
0.88
15.1
515° C.
1 mm in
depth from
head surface
57
6° C./sec
Pearlite
385
385
0.88
15.4
515° C.
1 mm in
depth from
head surface
58
6° C./sec
Pearlite
380
385
0.88
15.2
515° C.
1 mm in
depth from
head surface
59
2° C./sec
Pearlite
298
380
0.80
13.3
520° C.
3 mm in
depth from
head surface
60
12° C./sec
Pearlite
285
398
0.65
11.3
450° C.
2 mm in
depth from
head surface
61
7° C./sec
Pearlite
335
448
0.64
12.0
450° C.
2 mm in
depth from
head surface
62
7° C./sec
Pearlite
355
448
0.64
12.2
450° C.
2 mm in
depth from
head surface
63
7° C./sec
Pearlite
385
448
0.64
12.5
450° C.
2 mm in
depth from
head surface
TABLE 7
Time from
end of hot
rolling to
Rail
beginning
Hot rolling conditions
length
of
3
Time
2
Time
Time
Rolling
Classification
at hot
accelerated
passes
between
passes
between
1 pass
between
end
of
rolling
cooling
to
passes
to
passes
to
passes
Final
temperature
rail
Symbol
Steel
(m)
(sec)
final %
sec
final %
sec
final %
sec
pass %
° C.
Invented
64
35
135
115
18
7
3
1
7
920
rail
65
35
135
115
8
1
8
1
8
1
7
920
66
36
120
58
—
10
900
67
37
110
25
8
0.5
8
0.5
8
0.5
12
930
68
29
155
158
—
5
980
69
33
165
155
—
20
15
2
15
7
950
70
33
165
155
10
2
8
3
8
20
5
950
Accelerated
cooling
conditions
Number of
Hardness of
Tensile
of head
Microstructure
pearlite
head
test
portion
of head
blocks 1 to
portion
result of
Top: Cooling
portion
15 μm in
(5 mm in
Amount
head
rate
(5 mm in
grain size
depth from
of wear
portion
Classification
Bottom:
depth
(per 0.2 mm2)
head
of head
Total
of
Cooling end
from head
Measurement
surface)
portion
elongation
rail
Symbol
temperature
surface)
position
(Hv 10 kgf)
(g)
(%)
Invented
64
8° C./sec
Pearlite
398
462
0.50
10.8
rail
485° C.
3 mm in
depth from
head surface
65
8° C./sec
Pearlite
435
462
0.50
11.5
485° C.
3 mm in
depth from
head surface
66
12° C./sec
Pearilte
385
488
0.38
10.8
465° C.
2 mm in
depth from
head surface
67
18° C./sec
Pearlite
487
489
0.31
10.6
495° C.
3 mm in
depth from
head surface
68
6° C./sec
Pearlite
245
385
0.88
13.1
515° C.
1 mm in
(Small
depth from
area
head surface
reduction
ratio)
69
7° C./sec
Pearlite
265
448
0.64
11.0
450° C.
2 mm in
(Long time
depth from
between
head surface
passes)
70
7° C./sec
Pearlite
235
448
0.64
10.5
450° C.
2 mm in
(Small
depth from
area
head surface
reduction
ratio)
(Long time
between
passes)
Table 8 shows, regarding each of the steel rails according to the present invention, chemical composition, the value of CE calculated from the equation (1) composed of the chemical composition, the production conditions of a casting before rolling, the cooling method at the heat treatment of a rail, and the microstructure and the state of pro-eutectoid cementite structure formation at a web portion.
Tables 9 and 10 shows, regarding each of the comparative steel rails, chemical composition, the value of CE calculated from the equation (1) composed of the chemical composition, the production conditions of a casting before rolling, the cooling method at the heat treatment of a rail, and the microstructure and the state of pro-eutectoid cementite structure formation at a web portion.
Note that each of the steel rails listed in Tables 8, 9 and 10 was produced under the conditions of a time period of 180 sec. from hot rolling to heat treatment at the railhead portion and an area reduction ratio of 6% at the final pass of finish hot rolling.
In each of those rails, the number of the pearlite blocks having grain sizes in the range from 1 to 15 μm at a portion 5 mm in depth from the head top portion was in the range from 200 to 500 per 0.2 mm2 of observation field.
The rails listed in the tables are as follows:
Steel Rails According to the Present Invention (12 Rails), Symbols 71 to 82
The rails having the chemical composition in the aforementioned ranges, wherein the amount of formed pro-eutectoid cementite structures is reduced at the web portion of a rail, characterized in that the number of pro-eutectoid cementite network (NC) at a web portion does not exceed the value of CE calculated from the contents of the aforementioned chemical composition.
Comparative Steel Rails (11 Rails), Symbols 83 to 93
Symbols 83 to 88 (6 rails): the comparative steel rails, wherein the amounts of C, Si, Mn, P, S and Cr in alloying are outside the respective ranges according to the claims of the present invention.
Symbols 89 to 93 (5 rails): the comparative steel rails having the chemical composition in the aforementioned ranges, wherein the number of pro-eutectoid cementite network (NC) at a web portion exceeds the value of CE calculated from the contents of the aforementioned chemical composition.
Here, explanations are given regarding the drawings attached hereto. Reference numeral 5 (the region shaded with oblique lines) in
As seen in Tables 8, 9 and 10, in the cases of the steel rails according to the present invention in contrast to the cases of the comparative steel rails, the number of the pro-eutectoid cementite network (the number of intersecting cementite network, NC) forming at a web portion was reduced to the value of CE or less as a result of controlling the addition amounts of C, Si, Mn, P, S and Cr within the respective prescribed ranges.
In addition, the number of the pro-eutectoid cementite network (the number of intersecting cementite network, NC) forming at a web portion was reduced to the value of CE or less also as a result of optimizing the soft reduction during casting and applying cooling to the web portion.
As stated above, the number of the pro-eutectoid cementite network (the number of intersecting cementite network, NC) forming at a web portion was reduced to the value of CE or less as a result of controlling the addition amounts of C, Si, Mn, P, S and Cr within the respective prescribed ranges and, in addition, optimizing the soft reduction during casting and applying cooling to the web portion. Thus it was possible to prevent the deterioration of toughness at the web portion of a rail.
TABLE 8
Formation of
pro-eutectoid
cementite
structure in
web portion *3
Chemical composition (mass %)
Number of
Classi-
Mo/V/Nb/B/
Casting conditions and
pro-eutectoid
fication
Co/Cu/Ni/Ti/
CE
cooling method at rail
Microstructure of
cementite
of rail
Symbol
C
Si
Mn
P
S
Cr
Mg/Ca/Al/Zr/N
*1
heat treatment
web portion *2
network (NC)
Invented
71
0.86
0.25
1.02
0.015
0.010
0.21
N: 0.0085
20
Optimization of light
Pearlite + trace
16
rail
thickness reduction
pro-eutectoid
during casting
cementite
72
0.90
0.15
0.65
0.028
0.015
0.25
27
Optimization of light
Pearlite + trace
25
thickness reduction
pro-eutectoid
during casting
cementite
73
0.93
0.56
1.75
0.015
0.011
0.10
Ni: 0.20
25
Optimization of light
Pearlite + trace
20
thickness reduction
pro-eutectoid
during casting
cementite
74
0.95
0.80
0.11
0.011
0.010
0.78
26
Optimization of light
Pearlite + trace
21
thickness reduction
pro-eutectoid
during casting
cementite
75
0.98
0.40
0.70
0.018
0.024
0.25
26
Optimization of light
Pearlite + trace
22
thickness reduction
pro-eutectoid
during casting
cementite
76
1.00
1.35
0.45
0.012
0.008
0.15
Co: 0.15
8
Optimization of light
Pearlite + trace
5
Mo: 0.03
thickness reduction
pro-eutectoid
during casting
cementite
Cooling of web portion
77
1.05
0.50
1.00
0.008
0.010
0.35
Al: 0.10
29
Cooling of web portion
Pearlite + trace
27
Cu: 0.25
pro-eutectoid
cementite
78
1.10
1.25
0.65
0.010
0.015
0.12
Mg: 0.0015
15
Optimization of light
Pearlite + trace
10
Ca: 0.0015
thickness reduction
pro-eutectoid
during casting
cementite
Cooling of web portion
79
1.13
0.80
0.95
0.012
0.019
0.06
B: 0.0012
24
Cooling of web portion
Pearlite + trace
18
Ti: 0.0120
pro-eutectoid
cementite
80
1.15
0.70
0.45
0.012
0.009
0.15
Nb: 0.011
23
Cooling of web portion
Pearlite + trace
18
V: 0.02
pro-eutectoid
cementite
81
1.19
1.80
0.55
0.011
0.012
0.08
Zr: 0.0015
13
Optimization of light
Pearlite + trace
7
Al: 0.05
thickness reduction
pro-eutectoid
during casting
cementite
Cooling of web portion
82
1.35
1.51
0.35
0.012
0.012
0.15
26
Optimization of light
Pearlite + trace
22
thickness reduction
pro-eutectoid
during casting
cementite
Cooling of web portion
Note:
Balance of chemical composition is Fe and unavoidable impurities.
*1: CE = 60[mass % C] − 10[mass % Si] + 10[mass % Mn] + 500[mass % P] + 50[mass % S] + 30[mass % Cr] − 54
*2: Portion at the center of web centerline is observed with an optical microscope.
*3: Portion where pro-eutectoid cementite structures are exposed at the center of web centerline is observed with an optical microscope, and number of intersections of pro-eutectoid cementite network with two line segments each 300 μm in length crossing each other at right angles is counted under a magnification of 200 (see FIG. 2). Number of intersecting pro-eutectoid cementite network is defined as the total of the intersections on the two line segments.
TABLE 9
Formation of
Chemical composition (mass %)
pro-eutectoid cementite
Mo/V/Nb/
structure in web
Classi-
B/Co/Cu/
Casting conditions and
Microstructure
portion *3
fication
Ni/Ti/Mg/
CE
cooling method at rail
of
Number of pro-eutectoid
of rail
Symbol
C
Si
Mn
P
S
Cr
Ca/Al/Zr/
*1
heat treatment
web portion *2
cementite network (NC)
Com-
83
1.45
1.70
0.45
0.015
0.012
0.08
Zr: 0.0020
31
Optimization of light
Pearlite + trace
39
parative
Al: 0.04
thickness reduction
pro-eutectoid
Excessive segregation in
rail
during casting
cementite
web portions
Cooling of web portion
Excessive cementite
formation
84
1.00
2.51
0.51
0.015
0.015
0.25
Co: 0.25
2
Optimization of light
Pearlite + trace
2
thickness reduction
pro-eutectoid
during casting
cementite
Cooling of web portion
85
0.93
0.50
2.85
0.015
0.020
0.15
38
Optimization of light
Pearlite + trace
45
thickness reduction
pro-eutectoid
Excessive segregation in
during casting
cementite
web portion,
Excessive cementite
formation
86
0.90
0.25
0.68
0.035
0.015
0.25
30
Optimization of light
Pearlite + trace
35
thickness reduction
pro-eutectoid
Excessive segregation in
during casting
cementite
web portion,
Excessive cementite
formation
87
0.98
0.42
0.65
0.019
0.032
0.25
26
Optimization of light
Pearlite + trace
35
thickness reduction
pro-eutectoid
Excessive segregation in
during casting
cementite
web portion,
Excessive cementite
formation
88
0.95
0.75
0.15
0.012
0.015
1.25
41
Optimization of light
Pearlite + trace
58
thickness reduction
pro-eutectoid
Excessive segregation in
during casting
cementite
web portion,
Excessive cementite
formation
89
0.98
0.40
0.70
0.018
0.024
0.25
26
No control of light
Pearlite + trace
34
thickness reduction
pro-eutectoid
Excessive pro-eutectoid
during casting
cementite
cementite formation
No cooling of web
portion at heat
treatment
90
1.05
0.50
1.00
0.008
0.010
0.35
Al: 0.10
29
No control of light
Pearlite + trace
32
Cu: 0.25
thickness reduction
pro-eutectoid
Excessive pro-eutectoid
during casting
cementite
cementite formation
No cooling of web
portion at heat
treatment
Note:
Balance of chemical composition is Fe and unavoidable impurities.
*1: CE = 60[mass % C] − 10[mass % Si] + 10[mass % Mn] + 500[mass % P] + 50[mass % S] + 30[mass % Cr] − 54
*2: Portion at the center of web centerline is observed with an optical microscope.
*3: Portion where pro-eutectoid cementite structures are exposed at the center of web centerline is observed with an optical microscope, and number of intersections of pro-eutectoid cementite network with two line segments each 300 μm in length crossing each other at right angles is counted under a magnification of 200 (see FIG. 2). Number of intersecting pro-eutectoid cementite network is defined as the total of the intersections on the two line segments.
TABLE 10
Formation of
pro-eutectoid cementite
Chemical composition (mass %)
Casting conditions
structure in web
Classi-
Mo/V/Nb/B/
and cooling
portion *3
fication
Co/Cu/Ni/Ti/
CE
method at rail
Microstructure of
Number of pro-eutectoid
of rail
Symbol
C
Si
Mn
P
S
Cr
Mg/Ca/Al/Zr
*1
heat treatment
web portion *2
cementite network (NC)
Compara-
91
1.10
1.25
0.65
0.010
0.015
0.12
Mg: 0.0015
15
No control of light
Pearlite + trace
22
tive rail
Ca: 0.0015
thickness reduction
pro-eutectoid
Excessive pro-eutectoid
during casting
cemantite
cementite formation
No cooling of web
portion at heat
treatment
92
1.15
0.70
0.45
0.012
0.009
0.15
Nb: 0.011
23
No control of light
Pearlite + trace
28
v: 0.02
thickness reduction
pro-eutectoid
Excessive pro-eutectoid
during casting
cementite
cementite formation
No cooling of web
portion at heat
treatment
93
1.35
1.51
0.35
0.012
0.012
0.15
26
No control of light
Pearlite + trace
32
thickness reduction
pro-eutectoid
Excessive pro-eutectoid
during casting
cementite
cementite formation
No cooling of web
portion at heat
treatment
Note:
Balance of chemical composition is Fe and unavoidable impurities.
*1: CE = 60[mass % C] − 10[mass % Si] + 10[mass % Mn] + 500[mass % P] + 50[mass % S] + 30[mass % Cr] − 54
*2: Portion at the center of web centerline is observed with an optical microscope.
*3: portion where pro-eutectoid cementite structures are exposed at the center of web centerline is observed with an optical microscope, and number of intersections of pro-eutectoid cementite network with two line segments each 300 μm in length crossing each other at right angles is counted under a magnification of 200 (see FIG. 2). Number of intersecting pro-eutectoid cementite network is defined as the total of the intersections on the two line segments.
Table 11 shows the chemical composition of the steel rails subjected to the tests below. Note that the balance of the chemical composition specified in the table is Fe and unavoidable impurities.
Tables 12 and 13 show, regarding each of the rails produced by the production method according to the present invention using the steels listed in Table 11, the final rolling temperature, the rolling length, the time period from the end of rolling to the beginning of accelerated cooling, the conditions of accelerated cooling at the head, web and base portions of a rail, the microstructure, the number and the measurement position of pearlite blocks having grain sizes in the range from 1 to 15 μm, the result of drop weight test, the hardness at a head portion, and the value of total elongation in the tensile test of a head portion.
Tables 14 and 15 show, regarding each of the rails produced by comparative production methods using the steels listed in Table 11, the final rolling temperature, the rolling length, the time period from the end of rolling to the beginning of accelerated cooling, the conditions of accelerated cooling at the head, web and base portions of a rail, the microstructure, the number and the measurement position of pearlite blocks having grain sizes in the range from 1 to 15 μm, the result of drop weight test, the hardness at a head portion, and the value of total elongation in the tensile test of a head portion.
The rails listed in the tables are as follows:
Heat-treated Rails According to the Present Invention (11 rails), Symbols 94 to 104
The rails produced under the production conditions in the aforementioned ranges using the steels having the chemical composition in the aforementioned ranges.
Comparative Heat-treated Rails (8 Rails), Symbols 105 to 112
The rails produced under the production conditions outside the aforementioned ranges using the steels having chemical composition in the aforementioned ranges.
Note that each of the steel rails listed in Tables 12 to 15 were produced under the condition of an area reduction ratio of 6% at the final pass of finish hot rolling.
The tests were carried out under the following conditions:
Drop Weight Test
As seen in Tables 12 to 15, in the steel rails having high carbon contents as listed in Table 11, in the cases of the steel rails produced by the production method according to the present invention wherein accelerated cooling was applied to the head, web and base portions of a rail within a prescribed time period after the end of hot rolling, in contrast to the cases of the steel rails produced by comparative production methods, it was possible to suppress the formation of pro-eutectoid cementite structures and thus prevent the deterioration of fatigue strength and toughness.
In addition, as seen in Tables 12 to 15, it was possible to secure a good wear resistance at a railhead portion, the uniformity of the material quality of a rail in the longitudinal direction, and a good ductility at a railhead portion as a result of controlling the accelerated cooling rate at a railhead portion, optimizing a rolling length, and controlling a final rolling temperature.
As stated above, in a steel rail a having a high carbon content, it was made possible: to suppress the formation of pro-eutectoid cementite structures detrimental to the occurrence of fatigue cracks and brittle cracks by applying accelerated cooling to the head, web and base portions of the rail within a prescribed time period after the end of hot rolling in an attempt to suppress the formation of pro-eutectoid cementite structures in the head, web and base portions of the rail; and also to secure a good wear resistance at the railhead portion, the uniformity of the material quality of the rail in the longitudinal direction, and a good ductility at the railhead portion by optimally selecting an accelerated cooling rate at the railhead portion, a rail length at rolling, and a final rolling temperature.
TABLE 11
Chemical composition (mass %)
Si/Mn/Cr/Mo/V/Nb/B/Co/
Steel
C
Cu/Ni/Ti/Mg/Ca/Al/Zr/N
43
0.86
Si: 0.35
Mn: 1.00
44
0.90
Si: 0.25
Mn: 0.80
Mo: 0.02
45
0.95
Si: 0.81
Mn: 0.42
Cr: 0.54
46
1.00
47
1.00
Si: 0.55 Cu: 0.35
Mn: 0.69
Cr: 0.21
48
1.01
Si: 0.75 V: 0.030
Mn: 0.45 N: 0.010
Cr: 0.45
49
1.11
Si: 1.35 Zr: 0.0017
Mn: 0.31
Cr: 0.34
50
1.19
Si: 0.58 Al: 0.08
Mn: 0.58
Cr: 0.20
51
1.35
Si: 0.45 N: 0.0080
Mn: 0.35
Cr: 0.15
TABLE 12
Time from end
Accelerated cooling
Rolling end
of hot rolling
conditions *2
temperature
to beginning
Accelerated
of head
Rolling
of accelerated
Accelerated
cooling end
portion *1
length
cooling
cooling rate
temperature
Symbol
Steel
(° C.)
(m)
(sec)
(° C./sec)
(° C.)
Invented
94
43
1000
200
Head
200
1.0
640
production
portion
method
Web portion
200
1.5
645
Base
200
1.2
642
portion
95
44
980
200
Head
190
1.2
648
portion
Web portion
190
1.8
645
Base
190
1.8
632
portion
96
45
960
150
Head
185
2.0
630
portion
Web portion
165
2.5
605
Base
165
2.5
600
portion
97
45
960
125
Head
165
6.0
450
portion
Web portion
165
3.0
570
Base
165
4.5
560
portion
98
46
950
150
Head
145
8.0
450
portion
Web portion
145
3.0
560
Base
148
4.5
530
portion
99
47
950
150
Head
150
7.5
465
portion
Web portion
150
3.5
540
Base
150
5.0
530
portion
Total
elongation
Number of pearlite
in tensile
blocks 1 to 15 μm in
Drop weight
Hardness
test of
grain size
test *4
of head
head
Sym-
(per 0.2 mm2)
HT: Head tension
portion *5
portion *6
bol
Microstructure *3
Measurement position
BT: Base tension
(Hv)
(%)
Invented
94
Pearlite
215 (2 mm in depth
HT: No fracture
330
14.0
production
from head surface)
BT: No fracture
method
Pearlite
—
Pearlite
—
95
Pearlite
220 (2 mm in depth
HT: No fracture
320
13.0
from head surface)
BT: No fracture
Pearlite
—
Pearlite
—
96
Pearlite
235 (2 mm in depth
HT: No fracture
365
12.5
from head surface)
BT: No fracture
Pearlite
—
Pearlite
—
97
Pearlite
255 (2 mm in depth
HT: No fracture
435
13.4
from head surface)
BT: No fracture
Pearlite
—
Pearlite
—
98
Pearlite
215 (2 mm in depth
HT: No fracture
405
10.2
from head surface)
BT: No fracture
Pearlite
—
Pearlite
—
99
Pearlite
226 (2 mm in depth
HT: No fracture
440
10.5
from head surface)
BT: No fracture
Pearlite
—
Pearlite
—
*1: Rolling end temperature of head portion is surface temperature immediately after rolling.
*2: Cooling rates of head, web and base portions are average figures in the region 0 to 3 mm in depth at the positions specified in description.
*3: Microstructures of head, web and base portions are observed at a depth of 2 mm at the same positions as specified in above cooling rate measurement.
*4: Drop weight test method is specified in description.
*5: Hardness of head portion is measured at the same position of head portion as specified in above microstructure observation.
*6: Tensile test method is specified in description.
TABLE 13
Rolling end
Time from end
Accelerated cooling
temperature
of hot rolling
conditions *2
of
to beginning
Accelerated
head
Rolling
of accelerated
Accelerated
cooling end
portion *1
length
cooling
cooling rate
temperature
Symbol
Steel
(° C.)
(m)
(sec)
(° C./sec)
(° C.)
Invented
100
47
920
115
Head
150
7.5
445
production
portion
method
Web portion
150
3.5
540
Base
150
5.0
530
portion
101
48
900
150
Head
125
3.0
530
portion
Web portion
125
3.5
520
Base
125
4.0
520
portion
102
49
880
100
Head
75
8.0
425
portion
Web portion
70
4.5
510
Base
60
4.5
510
portion
103
50
870
110
Head
35
13.0
415
portion
Web portion
35
8.0
505
Base
35
9.5
500
portion
104
51
900
105
Head
10
23.0
452
portion
Web portion
10
8.0
515
Base
10
9.5
520
portion
Total
elongation
Number of pearlite
in tensile
blocks 1 to 15 μm
Drop weight
Hardness
test of
in grain size
test *4
of head
head
Sym-
(per 0.2 mm2)
HT: Head tension
portion *5
portion *6
bol
Microstructure *3
Measurement position
BT: Base tension
(Hv)
(%)
Invented
100
Pearlite
350 (2 mm in depth
HT: No fracture
445
11.8
production
from head surface)
BT: No fracture
method
Pearlite
—
Pearlite
—
101
Pearlite
230 (2 mm in depth
HT: No fracture
395
10.8
from head surface)
BT: No fracture
Pearlite
—
Pearlite
—
102
Pearlite
380 (2 mm in depth
HT: No fracture
401
10.4
from head surface)
BT: No fracture
Pearlite
—
Pearlite
—
103
Pearlite
400 (2 mm in depth
HT: No fracture
485
10.3
from head surface)
BT: No fracture
Pearlite
—
Pearlite
—
104
Pearlite
362 (2 mm in depth
HT: No fracture
465
10.0
from head surface)
BT: No fracture
Pearlite
—
Pearlite
—
*1: Rolling end temperature of head portion is surface temperature immediately after rolling.
*2: Cooling rates of head, web and base portions are average figures in the region 0 to 3 mm in depth at the positions specified in description.
*3: Microstructures of head, web and base portions are observed at a depth of 2 mm at the same positions as specified in above cooling rate measurement.
*4: Drop weight test method is specified in description.
*5: Hardness of head portion is measured at the same position of head portion as specified in above microstructure observation.
*6: Tensile test method is specified in description.
TABLE 14
Time from end
Accelerated cooling
Rolling end
of hot rolling
conditions *2
temperature
to beginning
Accelerated
of head
Rolling
of accelerated
Accelerated
cooling end
portion *1
length
cooling
cooling rate
temperature
Symbol
Steel
(° C.)
(m)
(sec)
(° C./sec)
(° C.)
Comparative
105
44
980
200
Head
190
4.5
648
production
portion
method
Web portion
190
13.0
645
Base
190
11.5
632
portion
106
45
960
150
Head
185
0.5
630
portion
Web portion
165
0.4
605
Base
165
0.5
600
portion
107
45
960
125
Head
165
18.0
450
portion
Web portion
165
3.0
570
Base
165
4.5
560
portion
108
47
830
150
Head
150
7.5
465
portion
Web portion
150
3.5
540
Base
150
5.0
530
portion
Number of
Total
pearlite blocks
elongation
1 to 15 μm in
in tensile
grain size
Drop weight
Hardness
test of
(per 0.2 mm2)
test *4
of head
head
Sym-
Measurement
HT: Head tension
portion *5
portion *6
bol
Microstructure *3
position
BT: Base tension
(Hv)
(%)
Comparative
105
Pearlite
235 (2 mm in
HT: No fracture
375
14.0
production
depth from head
BT: Fractured
method
surface)
(Martensite
Martensite + pearlite
—
formed)
Martensite + pearlite
—
106
Pro-
—
HT: Fracture
315
12.5
eutectoid
(Pro-eutectoid
cementite + pearlite
cementite
Pro-
—
formed)
eutectiod
BT: Fractured
cementite + pearlite
(Pro-eutectoid
Pro-
—
cementite
eutectiod
—
formed)
cementite + pearlite
107
Martensite + pearlite
—
HT: Fractured
545
6.4
(Martensite
(Martensite
Pearlite
—
formed)
formed, low
Pearlite
—
BT: No fracture
ductility)
108
Pro-
—
HT: Fractured
560
5.5
eutectoid
(Pro-eutectoid
(Martensite
cementite + pearlite
cementite
formed, low
Pro-
—
formed)
ductility)
eutectoid
BT: Fractured
cementite + pearlite
(Pro-eutectoid
Pro-
—
cementite
eutectiod
—
formed)
cementite + pearlite
*1: Rolling end temperature of head portion is surface temperature immediately after rolling.
*2: Cooling rates of head, web and base portions are average figures in the region 0 to 3 mm in depth at the positions specified in description.
*3: Microstructures of head, web and base portions are observed at a depth of 2 mm at the same positions as specified in above cooling rate measurement.
*4: Drop weight test method is specified in description.
*5: Hardness of head portion is measured at the same position of head portion as specified in above microstructure observation.
*6: Tensile test method is specified in description.
TABLE 15
Time from end
Accelerated cooling
Rolling end
of hot rolling
conditions *2
temperature
to beginning
Accelerated
of head
Rolling
of accelerated
Accelerated
cooling end
portion *1
length
cooling
cooling rate
temperature
Symbol
Steel
(° C.)
(m)
(sec)
(° C./sec)
(° C.)
Comparative
109
47
920
115
Head
150
7.5
445
production
portion
method
Web portion
150
3.5
685
Base
150
5.0
700
portion
110
48
900
250
Head
125
3.0
530
(Excessive
portion
rail
length)
Web portion
125
3.5
520
Base
125
4.0
520
portion
111
49
1080
100
Head
75
8.0
425
portion
Web portion
70
4.5
510
Base
60
4.5
510
portion
112
50
860
110
Head
350
13.0
415
portion
Web portion
350
8.0
505
Base
350
9.5
500
portion
Number of
Total
pearlite blocks
elongation
1 to 15 μm in
in tensile
grain size
Drop weight
Hardness
test of
(per 0.2 mm2)
test *4
of head
head
Sym-
Measurement
HT: Head tension
portion *5
portion *6
bol
Microstructure *3
position
BT: Base tension
(Hv)
(%)
Comparative
109
Pearlite
305 (2 mm in
HT: No fracture
445
11.8
production
depth from head
BT: Fractured
method
surface)
(Pro-eutectoid
Pro-
—
cementite
eutectoid
formed)
cementite + pearlite
Pro-
—
eutectoid
cementite + pearlite
110
Pearlite
215 (2 mm in
HT: No fracture
395
10.8
depth from head
BT: Fractured
surface)
(Pro-eutectoid
Pearlite
—
cementite
Trace pro-
—
formed)
eutectoid
cementite
at rail
ends + pearlite
111
Pearlite
120 (2 mm in
HT: No fracture
401
7.8
depth from head
BT: No fracture
(Pearlite
surface)
coarsened
Pearlite
—
→ low
Pearlite
—
ductility)
112
Pro-
—
HT: Fractured
435
7.8
eutectoid
(Pro-eutectoid
(Ce-
cementite + pearlite
cementite
mentite
Pro-
—
formed)
formed
eutectoid
BT: Fractured
→ low
cementite + pearlite
(Pro-eutectoid
ductility)
Pro-
—
cementite
eutectoid
formed)
cementite + pearlite
*1: Rolling end temperature of head portion is surface temperature immediately after rolling.
*2: Cooling rates of head, web and base portions are average figures in the region 0 to 3 mm in depth at the positions specified in description.
*3: Microstructures of head, web and base portions are observed at a depth of 2 mm at the same positions as specified in above cooling rate measurement.
*4: Drop weight test method is specified in description.
*5: Hardness of head portion is measured at the same position of head portion as specified in above microstructure observation.
*6: Tensile test method is specified in description.
Table 16 shows the chemical composition of the steel rails subjected to the tests below. Note that the balance of the chemical composition specified in the table is Fe and unavoidable impurities.
Table 17 shows the reheating conditions of the bloom (slab) (the values of CT and CM, the maximum heating temperatures of the bloom (slab) (Tmax) and the retention times during which the bloom (slab) are heated to 1,100° C. or higher (Mmax)) when the rails are produced by the production method according to the present invention using the steels listed in Table 11, and the properties during hot rolling and after the hot rolling (the surface properties of the rails thus produced during hot rolling and after the hot rolling, and the structures and the hardness of the surface layers of the head portions). The table also shows the wear test results of the rails produced by the production method according to the present invention. Table 18 shows the reheating conditions of the bloom (slab) (the values of CT and CM, the maximum heating temperatures of the bloom (slab) (Tmax) and the retention times during which the bloom (slab) are heated to 1,100° C. or higher (Mmax)) when the rails are produced by comparative production methods using the steels listed in Table 16, and the properties during hot rolling and after the rolling (the surface properties of the rails thus produced during hot rolling and after the hot rolling, and the structures and the hardness of the surface layers of the head portions). The table also shows the wear test results of the rails produced by comparative production methods.
Note that each of the steel rails listed in Tables 17 and 18 was produced under the conditions of a time period of 180 sec. from hot rolling to heat treatment at the railhead portion and an area reduction ratio of 6% at the final pass of finish hot rolling.
Here, explanations are given regarding the drawings attached hereto.
In
The rails listed in the tables are as follows:
Heat-treated Rails According to the Present Invention (11 Rails), Symbols 113 to 123
The bloom (slab) and rails produced by the production method in the aforementioned ranges using the steels having the chemical composition in the aforementioned ranges.
Comparative Heat-treated rails (8 Rails), Symbols 124 to 131
The bloom (slab) and rails produced by the production methods outside the aforementioned ranges using the steels having the chemical composition in the aforementioned ranges.
The tests were carried out under the following conditions:
Rolling Wear Test
As seen in Tables 17 and 18, in the cases of the rails produced under the reheating conditions in the aforementioned ranges in contrast to the cases of the rails produced under comparative reheating conditions: the cracks and breaks of a bloom (slab) during rolling were prevented as a result of optimizing the maximum heating temperature of the bloom (slab) and the time period during which the bloom (slab) was heated to a certain temperature or higher in the reheating process for hot rolling the bloom (slab) having a high carbon content as listed in Table 16 into rails; and the deterioration of wear resistance was prevented as a result of suppressing the decarburization at the outer surface layer of a rail and preventing the formation of pro-eutectoid ferrite structures. Thus, it was possible to produce high-quality rails efficiently.
TABLE 16
Chemical composition (mass %)
Si/Mn/Cr/Mo/V/Nb/B/Co/
Steel
C
Cu/Ni/Ti/Mg/Ca/Al/Zr/N
52
0.86
Si: 0.50
Mn: 1.05
53
0.90
Si: 0.50 Mo: 0.02
Mn: 1.05
Cr: 0.25
54
0.90
Si: 0.25
Mn: 0.65
Cr: 0.22
55
1.00
Si: 0.41
Mn: 0.70
Cr: 0.25
56
1.01
—
57
1.01
Si: 0.81 V: 0.03
Mn: 0.65 N: 0.0080
Cr: 0.55
58
1.11
Si: 0.45 Cu: 0.25
Mn: 0.51
Cr: 0.34
59
1.21
Si: 1.35 Zr: 0.0015
Mn: 0.15 Ca: 0.0020
Cr: 0.15
60
1.38
Si: 0.35 Al: 0.07
Mn: 0.12
TABLE 17
Reheating conditions of bloom
(slab) for rolling into rail
Properties of rail during and after hot rolling
Maximum heating
Retention time
Surface
Hardness
temperature of
at 1,100° C. or
condition
of head
Wear test
Value
Value
bloom (slab)
higher
during
surface
result *5
of
of
Tmax
Mmax
and after
Structure of head
layer *4
Wear amount
Symbol
Steel
CT *1
CM *2
(° C.)
(min)
hot rolling
surface layer *3
(Hv)
(mm)
Invented
113
52
1362
487
1325
415
No bloom (slab)
Pearlite
324
1.95
production
breakage or rail
method
cracking
114
53
1337
465
1305
402
No bloom (slab)
Pearlite
354
1.89
breakage or rail
cracking
115
54
1309
443
1280
385
No bloom (slab)
Pearlite
395
1.65
breakage or rail
cracking
116
55
1280
420
1270
375
No bloom (slab)
Pearlite
415
1.45
breakage or rail
cracking
117
55
1280
420
1250
345
No bloom (slab)
Pearlite
424
1.38
breakage or rail
cracking
118
56
1277
418
1245
365
No bloom (slab)
Pearlite
385
1.58
breakage or rail
cracking
119
57
1277
415
1275
395
No bloom (slab)
Pearlite
451
1.21
breakage or rail
cracking
120
57
1277
415
1245
325
No bloom (slab)
Pearlite
465
1.15
breakage or rail
cracking
121
58
1246
393
1240
350
No bloom (slab)
Pearlite
435
1.20
breakage or rail
cracking
122
59
1213
366
1200
315
No bloom (slab)
Pearlite
485
0.85
breakage or rail
cracking
123
60
1154
320
1140
300
No bloom (slab)
Pearlite
475
0.75
breakage or rail
cracking
*1 CT = 1500 − 140([mass % C]) − 80([mass % C])2
*2 CM = 600 − 120([mass % C]) − 60([mass % C])2
*3 Observation position of structure of head surface layer: 2 mm in depth from head top surface at rail width center
*4 Measurement position of hardness of head surface layer: 2 mm in depth from head top surface at rail width center
*5 Wear test method: See FIG. 9 and description. Wear amount: wear depth in height direction at rail width center after testing
TABLE 18
Reheating conditions of bloom
(slab) for rolling into rail
Properties of rail during and after hot rolling
Maximum heating
Retention time
Surface
Hardness
Wear test
temperature of
at 1,100° C. or
condition
of head
result *5
Value
Value
bloom (slab)
higher
during
surface
Wear
Sym-
of
of
Tmax
Mmax
and after
Structure of head
layer *4
amount
bol
Steel
CT *1
CM *2
(° C.)
(min)
hot rolling
surface layer *3
(Hv)
(mm)
Compar-
124
53
1337
465
1305
600
No bloom (slab)
Pearlite + pro-eutectoid
324
3.05
ative
breakage or rail
ferrite
production
cracking
(Much decarburization)
method
125
54
1309
443
1320
385
Rail cracked
Pearlite
385
1.75
126
55
1280
420
1300
485
Rail cracked
Pearlite + pro-eutectoid
365
2.85
ferrite
(Much decarburization)
127
55
1280
420
1355
345
Bloom (slab)
Hot rolling of rail not viable
broke
128
57
1277
415
1275
550
No bloom (slab)
Pearlite + pro-eutectoid
390
2.64
breakage or rail
ferrite
cracking
(Much decarburization)
129
58
1246
393
1220
500
No bloom (slab)
Pearlite + pro-eutectoid
398
2.45
breakage or rail
ferrite
cracking
(Much decarburization)
130
58
1213
366
1240
320
Rail cracked
Pearlite
475
0.91
131
60
1154
320
1250
300
Bloom (slab)
Hot rolling of rail not viable
broke
*1 CT = 1500 − 140([mass % C]) − 80([mass % C])2
*2 CM = 600 − 120([mass % C]) − 60([mass % C])2
*3 Observation position of structure of head surface layer: 2 mm in depth from head top surface at rail width center
*4 Measurement position of hardness of head surface layer: 2 mm in depth from head top surface at rail width center
*5 Wear test method: See FIG. 9 and description. Wear amount: wear depth in height direction at rail width center after testing
Table 19 shows the chemical composition of the steel rails subjected to the tests below. Note that the balance of the chemical composition specified in the table is Fe and unavoidable impurities.
Tables 20 and 21 show, regarding each of the rails produced by the heat treatment method according to the present invention using the steels listed in Table 19, the rolling length, the time period from the end of rolling to the beginning of the heat treatment of a base toe portion, the conditions of the accelerated cooling at the head, web and base portions of a rail, the microstructure, the result of a drop-weight test, and the hardness at a head portion.
Tables 22 and 23 show, regarding each of the rails produced by the comparative heat treatment methods using the steels listed in Table 19, the rolling length, the time period from the end of rolling to the beginning of the heat treatment of a base toe portion, the conditions of the accelerated cooling at the head, web and base portions of a rail, the microstructure, the result of a drop-weight test, and the hardness at a head portion.
The rails listed in the tables are as follows:
Heat-treated Rails According to the Present Invention (11 rails), Symbols 132 to 142
The rails produced under the heat treatment conditions in the aforementioned ranges using the steels having the chemical composition in the aforementioned ranges.
Comparative Heat-treated Rails (9 Rails), Symbols 143 to 151
The rails produced under the heat treatment conditions outside the aforementioned ranges using the steels having the chemical composition in the aforementioned ranges.
Note that each of the steel rails listed in Tables 20 and 21 was produced under the conditions of a time period of 180 sec. from hot rolling to heat treatment at the railhead portion and an area reduction ratio of 6% at the final pass of finish hot rolling.
In each of those rails, the number of the pearlite blocks having grain sizes in the range from 1 to 15 μm at a portion 5 mm in depth from the head top portion was in the range from 200 to 500 per 0.2 mm2 of observation field.
The tests were carried out under the following conditions:
Drop-weight Test
As seen in Tables 20 and 21, and 22 and 23, in the steel rails having high carbon contents as listed in Table 19, in the cases of the steel rails produced by the heat treatment method according to the present invention wherein preliminary heat treatment was applied to the base toe portion of a rail within the prescribed time period after the end of hot rolling and thereafter accelerated cooling was applied to the head, web and base portions, in contrast to the cases of the rails produced by the comparative production methods, the formation of pro-eutectoid cementite structures was suppressed and thus the deterioration of fatigue strength and toughness was prevented.
In addition, as shown in Tables 20 and 21, and 22 and 23, it was made possible to secure a good wear resistance at the railhead portions as a result of controlling the accelerated cooling rates at the railhead portions.
As stated above, in the steel rails having high carbon contents, it was made possible: to suppress the formation of pro-eutectoid cementite structures detrimental to the occurrence of fatigue cracks and brittle cracks as a result of applying accelerated cooling or heating to the base toe portions of a rail within the prescribed time period after the end of hot rolling and thereafter applying accelerated cooling to the head, web and base portions of the rail; and also to secure a good wear resistance at a railhead portion as a result of optimizing the accelerated cooling rate at the railhead portion.
TABLE 19
Chemical composition (mass %)
Si/Mn/Cr/Mo/V/Nb/B/Co/
Steel
C
Cu/Ni/Ti/Mg/Ca/Al/Zr/N
61
0.86
Si: 0.50
Mn: 0.80
62
0.90
Si: 0.35 Mo: 0.03
Mn: 0.80
63
0.95
Si: 0.80
Mn: 0.50
Cr: 0.45
64
1.00
65
1.00
Si: 0.55
Mn: 0.70
Cr: 0.25
66
1.01
Si: 0.80 V: 0.020
Mn: 0.45 N: 0.010
Cr: 0.40
67
1.11
Si: 1.45 Zr: 0.0020
Mn: 0.35 V: 0.050
Cr: 0.41
68
1.19
Si: 0.45 Al: 0.07
Mn: 0.65
Cr: 0.15
69
1.35
Si: 0.45 Cu: 0.15
Mn: 0.45
TABLE 20
Preliminary heat
treatment conditions
Time up to the start of heat treatment of
and microstructure of
Symbol
Steel
Rolling length (m)
base toe portion (sec)
base toe portion *1
Invented
132
61
198
58
Accelerated cooling
heat
rate: 5° C./sec.
treatment
Accelerated cooling end
method
temperature: 645° C.
Microstructure: pearlite
133
62
180
52
Accelerated cooling
rate: 6° C./sec.
Accelerated cooling end
temperature: 635° C.
Microstructure: pearlite
134
63
185
48
Accelerated cooling
rate: 7° C./sec.
Accelerated cooling end
temperature: 625° C.
Microstructure: pearlite
135
63
158
45
Heating by 56° C.
Microstructure: pearlite
136
64
168
40
Accelerated cooling
rate: 10° C./sec.
Accelerated cooling end
temperature: 615° C.
Microstructure: pearlite
137
65
178
40
Heating by 78° C.
Microstructure: pearlite
Accelerated cooling
conditions *2
Accelerated
Accelerated
Drop-weight
cooling
cooling end
test *4
Hardness of
rate
temperature
HT: Head tension
head portion *5
Symbol
Portion
(° C./sec)
(° C.)
Microstructure *3
BT: Base tension
(Hv)
Invented
132
Head
1.2
640
Pearlite
HT: No fracture
329
heat
portion
BT: No fracture
treatment
Web portion
1.5
642
Pearlite
method
Base
1.6
635
Pearlite
portion
133
Head
1.4
645
Pearlite
HT: No fracture
329
portion
BT: No fracture
Web portion
1.8
640
Pearlite
Base
1.8
630
Pearlite
portion
134
Head
2.4
625
Pearlite
HT: No fracture
385
portion
BT: No fracture
Web portion
2.6
615
Pearlite
Base
2.0
615
Pearlite
portion
135
Head
6.5
450
Pearlite
HT: No fracture
455
portion
BT: No fracture
Web portion
3.5
580
Pearlite
Base
4.0
550
Pearlite
portion
136
Head
6.0
485
Pearlite
HT: No fracture
420
portion
BT: No fracture
Web portion
3.0
530
Pearlite
Base
5.5
535
Pearlite
portion
137
Head
3.0
485
Pearlite
HT: No fracture
350
portion
BT: No fracture
Web portion
3.0
530
Pearlite
Base
5.5
535
Pearlite
portion
*1: Cooling rate of base toe portion is average figure in the region 0 to 3 mm in depth at the position specified in description.
*2: Cooling rates of head, web and base portions are average figures in the region 0 to 3 mm in depth at the positions specified in description.
*3: Microstructures of base toe, head, web and base portions are observed at a depth of 2 mm at the same positions as specified in above cooling rate measurement.
*4: Drop-weight test method is specified in description.
*5: Hardness of head portion is measured at same position of head portion as specified in above microstructure observation.
TABLE 21
Preliminary heat
treatment conditions
Time up to the start of heat treatment of
and microstructure of
Symbol
Steel
Rolling length (m)
base toe portion (sec)
base toe portion *1
Invented
138
65
160
40
Heating by 85° C.
heat
Microstructure: pearlite
treatment
method
139
66
155
35
Accelerated cooling
rate: 12° C./sec.
Accelerated cooling end
temperature: 545° C.
Microstructure: pearlite
140
67
145
25
Heating by 95° C.
Microstructure: pearlite
141
68
125
10
Accelerated cooling
rate: 17° C./sec.
Accelerated cooling end
temperature: 545° C.
Microstructure: pearlite
142
69
105
10
Accelerated cooling
rate: 20° C./sec.
Accelerated cooling end
temperature: 525° C.
Microstructure: pearlite
Accelerated cooling
conditions *2
Accelerated
Accelerated
Drop-weight
cooling
cooling end
test *4
Hardness of
rate
temperature
HT: Head tension
head portion *5
Symbol
Portion
(° C./sec)
(° C.)
Microstructure *3
BT: Base tension
(Hv)
Invented
138
Head
7.0
440
Pearlite
HT: No fracture
435
heat
portion
BT: No fracture
treatment
Web portion
3.5
545
Pearlite
method
Base
5.5
525
Pearlite
portion
139
Head
3.5
530
Pearlite
HT: No fracture
385
portion
BT: No fracture
Web portion
3.5
520
Pearlite
Base
4.5
520
Pearlite
portion
140
Head
8.5
445
Pearlite
HT: No fracture
425
portion
BT: No fracture
Web portion
4.0
530
Pearlite
Base
4.0
525
Pearlite
portion
141
Head
12.0
425
Pearlite
HT: No fracture
475
portion
BT: No fracture
Web portion
7.0
515
Pearlite
Base
9.0
505
Pearlite
portion
142
Head
20.0
430
Pearlite
HT: No fracture
495
portion
BT: No fracture
Web portion
7.0
505
Pearlite
Base
9.0
510
Pearlite
portion
*1: Cooling rate of base toe portion is average figure in the region 0 to 3 mm in depth at the position specified in description.
*2: Cooling rates of head, web and base portions are average figures in the region 0 to 3 mm in depth at the positions specified in description.
*3: Microstructures of base toe, head, web and base portions are observed at a depth of 2 mm at the same positions as specified in above cooling rate measurement.
*4: Drop-weight test method is specified in description.
*5: Hardness of head portion is measured at same position of head portion as specified in above microstructure observation.
TABLE 22
Preliminary heat treatment
Time up to the start of heat treatment of
conditions and microstructure of
Symbol
Steel
Rolling length (m)
base toe portion (sec)
base toe portion *1
Comparative
143
62
180
52
Accelerated cooling
heat
rate: 5° C./sec.
treatment
Accelerated cooling end
method
temperature: 700° C.
Microstructure: pro-
eutectoid cementite + pearlite
144
63
185
48
Accelerated cooling
rate: 25° C./sec.
Accelerated cooling end
temperature: 625° C.
Microstructure:
martensite + pearlite
145
63
158
45
Heating by 56° C.
Microstructure:
martensite + pearlite
146
65
178
40
Heating by 15° C.
Microstructure: pro-
eutectoid cementite + pearlite
147
65
160
40
Heating by 85° C.
Microstructure: pearlite
Accelerated cooling
conditions *2
Accelerated
Accelerated
Drop-weight
cooling
cooling end
test *4
Hardness of
rate
temperature
HT: Head tension
head portion *5
Symbol
Portion
(° C./sec)
(° C.)
Microstructure *3
BT: Base tension
(Hv)
Comparative
143
Head
1.4
645
Pearlite
HT: No fracture
329
heat
portion
BT: Fractured
treatment
Web portion
1.8
640
Pearlite
(Pro-eutectoid
method
Base
1.8
630
Pearlite
cementite
portion
formed)
144
Head
2.4
625
Pearlite
HT: No fracture
375
portion
BT: Fractured
Web portion
2.6
615
Pearlite
(Martensite
Base
2.0
615
Pearlite
formed)
portion
145
Head
6.5
450
Pearlite
HT: No fracture
445
portion
BT: Fractured
Web portion
12.5
580
Martensite + pearlite
(Martensite
Base
13.0
550
Martensite + pearlite
formed)
portion
146
Head
17.0
485
Martensite + pearlite
HT: Fractured
514
portion
(Martensite
Web portion
3.0
530
Pearlite
formed)
Base
5.5
535
Pearlite
BT: Fractured
portion
(Pro-eutectoid
cementite
formed)
147
Head
0.5
550
Pro-
HT: Fractured
425
portion
eutectoid
(Pro-eutectoid
cementite + pearlite
cementite
Web portion
0.5
545
Pro-
formed)
eutectoid
BT: Fractured
cementite + pearlite
(Pro-eutectoid
Base
0.5
525
Pro-
cementite
portion
eutectoid
formed)
cementite + pearlite
*1: Cooling rate of base toe portion is average figure in the region 0 to 3 mm in depth at the position specified in description.
*2: Cooling rates of head, web and base portions are average figures in the region 0 to 3 mm in depth at the positions specified in description.
*3: Microstructures of base toe, head, web and base portions are observed at a depth of 2 mm at the same positions as specified in above cooling rate measurement.
*4: Drop-weight test method is specified in description.
*5: Hardness of head portion is measured at same position of head portion as specified in above microstructure observation.
TABLE 23
Preliminary heat
treatment conditions
Time up to the start of heat treatment
and microstructure of
Symbol
Steel
Rolling length (m)
of base toe portion (sec)
base toe portion *1
Comparative
148
66
155
35
Accelerated cooling
heat
rate: 1° C./sec.
treatment
Accelerated cooling end
method
temperature: 545° C.
Microstructure: pro-
eutectoid cementite + pearlite
149
66
245
35
Accelerated cooling
(Excessive
rate: 12° C./sec.
rail
Accelerated cooling end
length)
temperature: 545° C.
Microstructure: pro-
eutectoid cementite + pearlite
150
67
145
25
Heating by 150° C.
Microstructure: coarse
pearlite
151
69
155
80
Accelerated cooling
rate: 20° C./sec.
Accelerated cooling end
temperature: 525° C.
Microstructure: pro-
eutectoid cementite
Accelerated cooling
conditions *2
Accelerated
Accelerated
Drop-weight
cooling
cooling end
test *4
Hardness of
rate
temperature
HT: Head tension
head portion *5
Symbol
Portion
(° C./sec)
(° C.)
Microstructure *3
BT: Base tension
(Hv)
Comparative
148
Head
3.5
530
Pearlite
HT: No fracture
385
heat
portion
BT: Fractured
treatment
Web portion
3.5
520
Pearlite
(Pro-eutectoid
method
Base
4.5
520
Pearlite
cementite
portion
formed)
149
Head
6.5
530
Pearlite
HT: No fracture
425
portion
BT: Fractured
Web portion
3.5
520
Pearlite
(Trace pro-
Base
5.5
520
Pearlite
eutectoid
portion
cementite
formed)
150
Head
8.5
445
Pearlite
HT: No fracture
425
portion
BT: Fractured
Web portion
4.0
530
Pearlite
(Pearlite
Base
4.0
525
Pearlite
coarsened)
portion
151
Head
20.0
430
Pearlite
HT: No fracture
495
portion
BT: Fractured
Web portion
7.0
505
Pearlite
(Pro-eutectoid
Base
9.0
510
Pearlite
cementite
portion
formed)
*1: Cooling rate of base toe portion is average figure in the region 0 to 3 mm in depth at the position specified in description.
*2: Cooling rates of head, web and base portions are average figures in the region 0 to 3 mm in depth at the positions specified in description.
*3: Microstructures of base toe, head, web and base portions are observed at a depth of 2 mm at the same positions as specified in above cooling rate measurement.
*4: Drop-weight test method is specified in description.
*5: Hardness of head portion is measured at same position of head portion as specified in above microstructure observation.
Table 24 shows the chemical composition of the steel rails subjected to the tests below. Note that the balance of the chemical composition specified in the table is Fe and unavoidable impurities. Tables 25 and 26 show, regarding each of the rails produced by the heat treatment method according to the present invention using the steels listed in Table 24, the rolling length, the time period from the end of rolling to the beginning of the heat treatment of a web portion, the heat treatment conditions and the microstructure of a web portion, the accelerated cooling conditions and the microstructures of the head and base portions of a rail, the number of intersecting pro-eutectoid cementite network (N) in a web portion, and the hardness at a head portion.
Tables 27, 28 and 29 show, regarding each of the rails produced by comparative heat treatment methods using the steels listed in Table 24, the rolling length, the time period from the end of rolling to the beginning of the heat treatment of a web portion, the heat treatment conditions and the microstructure of a web portion, the accelerated cooling conditions and the microstructures of the head and base portions of a rail, the number of intersecting pro-eutectoid cementite network (N) in a web portion, and the hardness at a head portion.
The rails listed in the tables are as follows:
Heat-treated Rails According to the Present Invention (11 rails), Symbols 152 to 162
The rails produced under the heat treatment conditions in the aforementioned ranges using the steels having the chemical composition in the aforementioned ranges.
Comparative Heat-treated Rails (11 Rails), Symbols 163 to 173
The rails produced under the heat treatment conditions outside the aforementioned ranges using the steels having the chemical composition in the aforementioned ranges.
Note that each of the steel rails listed in Tables 25 and 26, and 27, 28 and 29 were produced under the conditions of a time period of 180 sec. from hot rolling to heat treatment at the railhead portion and an area reduction ratio of 6% at the final pass of finish hot rolling.
In each of those rails, the number of the pearlite blocks having grain sizes in the range from 1 to 15 μm at a portion 5 mm in depth from the head top portion was in the range from 200 to 500 per 0.2 mm2 of observation field.
Here, explanations are given regarding the number of intersecting pro-eutectoid cementite network (N) mentioned in this example and the method for exposing pro-eutectoid cementite structures for the measurement thereof.
Firstly, the method for exposing pro-eutectoid cementite structures is explained. First, a cross-sectional surface of the web portion of a rail is polished with diamond abrasive. Then, the polished surface is immersed in a solution of picric acid and caustic soda and pro-eutectoid cementite structures are exposed. Some adjustments may be required of the exposing conditions in accordance with the condition of a polished surface, but, basically, desirable exposing conditions are: an immersion solution temperature is 80° C.; and an immersion time is approximately 120 min.
Secondly, the method for measuring the number of intersecting pro-eutectoid cementite network (N) is explained.
An arbitrary point where pro-eutectoid cementite structures are exposed on a sectional surface of the web portion of a rail is observed with an optical microscope. The number of intersections of pro-eutectoid cementite network with two line segments each 300 μm in length crossing each other at right angles is counted under a magnification of 200.
The number of the intersecting pro-eutectoid cementite network is defined as the total of the intersections on the two line segments each 300 μm in length crossing each other at right angles. Note that, in consideration of uneven distribution of pro-eutectoid cementite structures, it is desirable to carry out the counting at least at 5 observation fields and use the average of the counts as the representative figure of the specimen.
The results are shown in Tables 25 and 26, and 28 and 29. In the high carbon steel rails having the chemical composition listed in Table 24, in the cases of the steel rails produced by the heat treatment method according to the present invention wherein the heat treatment in the aforementioned ranges was applied to the web portion of a rail within the prescribed time period after the end of hot rolling and additionally the accelerated cooling in the aforementioned ranges was applied to the head and base portions of the rail, in contrast to the cases of the rails produced by comparative heat treatment methods, the numbers of intersecting pro-eutectoid cementite network (N) were significantly reduced.
In addition, in the cases of the steel rails produced by the heat treatment method according to the present invention wherein the accelerated cooling in the aforementioned ranges was applied, in contrast to the rails produced by the comparative heat treatment methods, it was possible to prevent the formation of martensite structures and coarse pearlite structures, which caused the deterioration of the toughness and the fatigue strength at the web portion of a rail, as a result of adequately controlling the cooling rates during the heat treatment.
In addition, as shown in Tables 25 and 26, and 28 and 29, a good wear resistance was secured at the railhead portions, as evidenced by the rails produced by the heat treatment method according to the present invention (Symbols 155 and 158 to 162), as a result of controlling the accelerated cooling rates at the railhead portions.
As stated above, in the steel rails having high carbon contents, it was made possible: to suppress the formation of pro-eutectoid cementite structures, which acted as the origins of brittle fracture and deteriorated fatigue strength and toughness, as a result of applying accelerated cooling or heating to the web portion of a rail within the prescribed time period after the end of hot rolling and also applying accelerated cooling to the head and base portions of the rail and, after heating of the web portion too; and, further, to secure a good wear resistance at a railhead portion as a result of optimizing the accelerated cooling rate at the railhead portion.
TABLE 24
Chemical composition (mass %)
Si/Mn/Cr/Mo/V/Nb/B/Co/
Steel
C
Cu/Ni/Ti/Mg/Ca/Al/Zr/N
70
0.86
Si: 0.25
Mn: 0.80
71
0.90
Si: 0.25 Cu: 0.25
Mn: 0.80
Cr: 0.20
72
0.95
Si: 0.80 Mo: 0.03
Mn: 0.50
Cr: 0.25
73
1.00
74
1.00
Si: 0.55
Mn: 0.65
Cr: 0.25
75
1.01
Si: 0.80 V: 0.02
Mn: 0.45 N: 0.0080
Cr: 0.40
76
1.11
Si: 1.45 Zr: 0.0015
Mn: 0.25
Cr: 0.35
77
1.19
Si: 0.85 Al: 0.08
Mn: 0.15
78
1.34
Si: 0.85
Mn: 0.15
TABLE 25
Time up to the
start of
heat
treatment
of web
Rolling length
portion
Heat treatment conditions and
Symbol
Steel
(m)
(sec)
microstructure of web portion *1
Invented
152
70
200
98
Accelerated cooling
Cooling
heat
rate: 2.0° C./sec.
treatment
Cooling end
method
temperature: 635° C.
Microstructure: pearlite
153
71
198
90
Accelerated cooling
Cooling
rate: 2.5° C./sec.
Cooling end
temperature: 645° C.
Microstructure: pearlite
154
72
185
88
Accelerated cooling
Cooling
rate: 3.8° C./sec.
Cooling end
temperature: 630° C.
Microstructure: pearlite
155
72
185
82
Heating
Cooling
25° C.
rate: 1.5° C./sec.
Cooling end
temperature: 642° C.
Microstructure: pearlite
156
73
180
80
Heating
Cooling
46° C.
rate: 3.5° C./sec.
Cooling end
temperature: 620° C.
Microstructure: pearlite
Formation of pro-
Accelerated cooling conditions
eutectoid cementite
and microstructure of head and
structure in web
base portions *2*3
portion *4
Accelerated
Accelerated
Number of
Hardness
cooling
cooling and
intersecting pro-
of head
rate
temperature
eutectoid cementite
portion *5
Symbol
Portion
(° C./sec)
(° C.)
Microstructure
network (N)
(Hv)
Invented
152
Head
1.4
640
Pearlite
Segregated
1
305
heat
portion
portion
treatment
Base
1.3
640
Pearlite
Surface
0
method
portion
layer
153
Head
1.5
645
Pearlite
Segregated
2
315
portion
portion
Base
1.6
640
Pearlite
Surface
0
portion
layer
154
Head
2.9
632
Pearlite
Segregated
5
332
portion
portion
Base
2.8
625
Pearlite
Surface
0
portion
layer
155
Head
4.9
475
Pearlite
Segregated
4
405
portion
portion
Base
4.5
635
Pearlite
Surface
1
portion
layer
156
Head
3.2
605
Pearlite
Segregated
6
360
portion
portion
Base
2.8
620
Pearlite
Surface
0
portion
layer
*1: Heating temperature, accelerated cooling rate, and accelerated cooling end temperature of web portion are average figures in the region 0 to 3 mm in depth at the positions specified in description.
*2: Accelerated cooling rates of head and base portions are average figures in the region 0 to 3 mm in depth at the positions specified in description.
*3: Microstructure of head, web and base portions are observed at a depth of 2 mm at the same positions as specified in above cooling rate measurement.
*4: See description and FIG. 2 for methods of exposing pro-eutectoid cementite structures and measuring the number of intersecting pro-eutectoid cementite network (N). N at segregated portion of web is measured at width center of rail centerline on cross-sectional surface of web portion. N at surface layer of web portion is measured at a depth of 2 mm at the same position as specified in above microstructure observation.
*5: Hardness of head portion is measured at the same position of head portion as specified in above microstructure observation.
TABLE 26
Time up to the
start of
heat
treatment
of web
Rolling length
portion
Heat treatment conditions and
Symbol
Steel
(m)
(sec)
microstructure of web portion *1
Invented
157
74
170
75
Heating
Cooling
heat
56° C.
rate: 2.8° C./sec.
treatment
Cooling end
method
temperature: 615° C.
Microstructure: pearlite
158
74
170
52
Heating
Cooling
74
rate: 4.0° C./sec.
Cooling end
temperature: 585° C.
Microstructure: pearlite
159
75
160
65
Accelerated cooling
Cooling
rate: 6.5° C./sec.
Cooling end
temperature: 545° C.
Microstructure:
pearlite
160
76
145
25
Heating
Cooling
98° C.
rate: 9.0° C./sec.
Cooling end
temperature: 525° C.
Microstructure:
pearlite
161
77
120
18
Accelerated
Cooling
cooling
rate: 16.0° C./sec.
Cooling end
temperature: 515° C.
Microstructure:
pearlite
162
78
105
10
Accelerated
Cooling
cooling
rate: 20.0° C./sec.
Cooling end
temperature: 535° C.
Microstructure:
pearlite
Formation of pro-
Accelerated cooling conditions
eutectoid cementite
and microstructure of head and
structure in web
base portions *2*3
portion *4
Accelerated
Accelerated
Number of
Hardness
cooling
cooling and
intersecting pro-
of head
rate
temperature
eutectoid cementite
portion *5
Symbol
Portion
(° C./sec)
(° C.)
Microstructure
network (N)
(Hv)
Invented
157
Head
2.8
595
Pearlite
Segregated
8
374
heat
portion
portion
treatment
Base
2.4
610
Pearlite
Surface
0
method
portion
layer
158
Head
7.0
480
Pearlite
Segregated
6
442
portion
portion
Base
4.5
545
Pearlite
Surface
0
portion
layer
159
Head
5.5
530
Pearlite
Segregated
7
378
portion
portion
Base
4.6
520
Pearlite
Surface
0
portion
layer
160
Head
11.0
445
Pearlite
Segregated
9
485
portion
portion
Base
6.0
535
Pearlite
Surface
1
portion
layer
161
Head
15.0
425
Pearlite
Segregated
8
455
portion
portion
Base
7.0
505
Pearlite
Surface
1
portion
layer
162
Head
18.0
435
Pearlite
Segregated
9
476
portion
portion
Base
10.0
521
Pearlite
Surface
1
portion
layer
*1: Heating temperature, accelerated cooling rate, and accelerated cooling end temperature of web portion are average figures in the region 0 to 3 mm in depth at the positions specified in description.
*2: Accelerated cooling rates of head and base portions are average figures in the region 0 to 3 mm in depth at the positions specified in description.
*3: Microstructure of head, web and base portions are observed at a depth of 2 mm at the same positions as specified in above cooling rate measurement.
*4: See description and FIG. 2 for methods of exposing pro-eutectoid cementite structures and measuring the number of intersecting pro-eutectoid cementite network (N). N at segregated portion of web is measured at width center of rail centerline on cross-sectional surface of web portion. N at surface layer of web portion is measured at a depth of 2 mm at the same position as specified in above microstructure observation.
*5: Hardness of head portion is measured at the same position of head portion as specified in above microstructure observation.
TABLE 27
Time
up to the start of
heat
treatment
of web
portion
Heat treatment conditions and
Symbol
Steel
Rolling length (m)
(sec)
microstructure of web portion *1
Comparative
163
71
198
90
Accelerated cooling
Cooling
heat
rate: 2.0° C./sec.
treatment
Cooling end
method
temperature: 720° C.
Microstructure:
pro-eutectoid
cementite + pearlite
164
72
185
88
Accelerated cooling
Cooling
rate: 24.0° C./sec.
Cooling end
temperature: 630° C.
Microstructure:
martensite + pearlite
165
72
185
82
Heating
Cooling
25° C.
rate: 13.0° C./sec.
Cooling end
temperature: 565° C.
Microstructure:
martensite + pearlite
166
74
170
75
Heating
Cooling
56° C.
rate: 0.5° C./sec.
Cooling end
temperature: 610° C.
Microstructure:
pro-eutectoid
cementite + pearlite
Formation of pro-
Accelerated cooling conditions
eutectoid cementite
and microstructure of head and
structure in web
base portions *2*3
portion *4
Accelerated
Accelerated
Number of
Hardness
cooling
cooling end
intersecting pro-
of head
rate
temperature
eutectoid cementite
portion *5
Symbol
Portion
(° C./sec)
(° C.)
Microstructure
network (N)
(Hv)
Comparative
163
Head
1.4
640
Pearlite
Segregated
21
320
heat
portion
portion
treatment
Base
1.5
645
Pearlite
Surface
8
method
portion
layer
164
Head
2.7
630
Pearlite
Segregated
3
335
portion
portion
Base
2.5
620
Pearlite
Surface
0
portion
layer
165
Head
4.7
470
Pearlite
Segregated
2
402
portion
portion
Base
4.6
630
Pearlite
Surface
0
portion
layer
166
Head
0.7
590
Pro-
Segregated
29
334
portion
eutectoid
portion
cementite + pearlite
Base
0.8
620
Pro-
Surface
8
portion
eutectoid
layer
cementite + pearlite
*1: Heating temperature, accelerated cooling rate, and accelerated cooling end temperature of web portion are average figures in the region 0 to 3 mm in depth at the positions specified in description.
*2: Accelerated cooling rates of head and base portions are average figures in the region 0 to 3 mm in depth at the positions specified in description.
*3: Microstructure of head, web and base portions are observed at a depth of 2 mm at the same positions as specified in above cooling rate measurement.
*4: See description and FIG. 2 for methods of exposing pro-eutectoid cementite structures and measuring the number of intersecting pro-eutectoid cementite network (N). N at segregated portion of web is measured at width center of rail centerline on cross-sectional surface of web portion. N at surface layer of web portion is measured at a depth of 2 mm at the same position as specified in above microstructure observation.
*5: Hardness of head portion is measured at the same position of head portion as specified in above microstructure observation.
TABLE 28
Time
up to the start of
heat
treatment
of web
portion
Heat treatment conditions and
Symbol
Steel
Rolling length (m)
(sec)
microstructure of web portion *1
Comparative
167
74
170
52
Heating
Cooling
heat
12° C.
rate: 4.2° C./sec.
treatement
Cooling end
method
temperature: 585° C.
Microstructure:
pro-eutectoid
cementite + pearlite
168
74
170
—
Heating
Natural cooling in
54° C.
air
Microstructure:
pro-eutectoid
cementite + pearlite
169
75
160
65
Accelerated cooling
Cooling
rate: 1.0° C./sec.
Cooling end
temperature: 550° C.
Microstructure:
pro-eutectoid
cementite + pearlite
170
75
235
35
Accelerated cooling
Cooling
(Excessive
rate: 3.5° C./sec.
rail
Cooling end
length)
temperature: 540° C.
Microstructure:
trace pro-
eutectoid
cementite at rail
ends + pearlite
Formation of pro-
Accelerated cooling conditions
eutectoid cementite
and microstructure of head and
structure in web
base portions *2*3
portion *4
Accelerated
Accelerated
Number of
Hardness
cooling
cooling end
intersecting pro-
of head
rate
temperature
eutectoid cementite
portion *5
Symbol
Portion
(° C./sec)
(° C.)
Microstructure
network (N)
(Hv)
Comparative
167
Head
7.2
485
Pearlite
Segregated
35
442
heat
portion
portion
treatement
Base
4.0
550
Pearlite
Surface
10
method
portion
layer
168
Head
7.2
485
Pearlite
Segregated
39
442
portion
portion
Base
Natural cooling in air
Pro-
Surface
20
portion
eutectoid
layer
cementite + pearlite
169
Head
5.0
535
Pearlite
Segregated
34
378
portion
portion
Base
4.5
525
Pearlite
Surface
11
portion
layer
170
Head
5.0
535
Pearlite
Segregated
25
388
portion
portion
Base
4.5
525
Pearlite
Surface
4
portion
layer
*1: Heating temperature, accelerated cooling rate, and accelerated cooling end temperature of web portion are average figures in the region 0 to 3 mm in depth at the positions specified in description.
*2: Accelerated cooling rates of head and base portions are average figures in the region 0 to 3 mm in depth at the positions specified in description.
*3: Microstructure of head, web and base portions are observed at a depth of 2 mm at the same positions as specified in above cooling rate measurement.
*4: See description and FIG. 2 for methods of exposing pro-eutectoid cementite structures and measuring the number of intersecting pro-eutectoid cementite network (N). N at segregated portion of web is measured at width center of rail centerline on cross-sectional surface of web portion. N at surface layer of web portion is measured at a depth of 2 mm at the same position as specified in above microstructure observation.
*5: Hardness of head portion is measured at the same position of head portion as specified in above microstructure observation.
TABLE 29
Time
up to the start of
heat
treatment
of web
portion
Heat treatment conditions and
Symbol
Steel
Rolling length (m)
(sec)
microstructure of web portion *1
Comparative
171
76
145
25
Heating
Cooling
heat
165° C.
rate: 9.0° C./sec.
tratement
Cooling end
method
temperature: 525° C.
Microstructure:
coarse pearlite
172
77
120
125
Accelerated cooling
Cooling
rate: 16.0° C./sec.
Cooling end
temperature: 515° C.
Microstructure:
pro-eutectoid
cementite + pearlite
173
78
105
—
Accelerated cooling
Natural cooling in
air
Microstructure:
pro-eutectoid
cementite + pearlite
Formation of pro-
Accelerated cooling conditions
eutectoid cementite
and microstructure of head and
structure in web
base portions *2*3
portion *4
Accelerated
Accelerated
Number of
Hardness
cooling
cooling end
intersecting pro-
of head
rate
temperature
eutectoid cementite
portion *5
Symbol
Portion
(° C./sec)
(° C.)
Microstructure
network (N)
(Hv)
Comparative
171
Head
12.5
445
Pearlite
Segregated
9
485
heat
portion
portion
tratement
Base
5.0
535
Pearlite
Surface
1
method
portion
layer
172
Head
18.0
455
Pearlite
Segregated
38
465
portion
portion
Base
6.0
505
Pearlite
Surface
14
portion
layer
173
Head
Natural cooling in air
Pro-
Segregated
40
345
portion
eutectoid
portion
cementite + pearlite
Base
Natural cooling in air
Pro-
Surface
24
portion
eutectoid
layer
cementite + pearlite
*1: Heating temperature, accelerated cooling rate, and accelerated cooling end temperature of web portion are average figures in the region 0 to 3 mm in depth at the positions specified in description.
*2: Accelerated cooling rates of head and base portions are average figures in the region 0 to 3 mm in depth at the positions specified in description.
*3: Microstructure of head, web and base portions are observed at a depth of 2 mm at the same positions as specified in above cooling rate measurement.
*4: See description and FIG. 2 for methods of exposing pro-eutectoid cementite structures and measuring the number of intersecting pro-eutectoid cementite network (N). N at segregated portion of web is measured at width center of rail centerline on cross-sectional surface of web portion. N at surface layer of web portion is measured at a depth of 2 mm at the same position as specified in above microstructure observation.
*5: Hardness of head portion is measured at the same position of head portion as specified in above microstructure observation.
Table 30 shows the chemical composition of the steel rails subjected to the tests below. Note that the balance of the chemical composition specified in the table is Fe and unavoidable impurities.
Tables 31 and 32 show the values of CCR of the steels listed in Table 30, and, regarding each of the rails produced through the heat treatment according to the present invention using the steels listed in Table 30, the rolling length, the time period up to the beginning of heat treatment, the heat treatment conditions (cooling rates and the values of TCR) at the inside and the surface of a railhead portion, and the microstructure of a railhead portion.
Tables 33 and 34 show the values of CCR of the steels listed in Table 30, and, regarding each of the rails produced through the comparative heat treatment using the steels listed in Table 30, the rolling length, the time period up to the beginning of heat treatment, the heat treatment conditions (cooling rates and the values of TCR) at the inside and the surface of a railhead portion, and the microstructure of a railhead portion.
Here, explanations are given regarding the drawings attached hereto.
In
The rails listed in the tables are as follows:
Heat-treated Rails According to the Present Invention (11 Rails), Symbols 174 to 184
The rails produced by applying heat treatment to the railhead portions under the conditions in the aforementioned ranges using the steels having the chemical composition in the aforementioned ranges.
Comparative Heat-treated Rails (10 Rails), Symbols 185 to 194
The rails produced by applying heat treatment to the railhead portions under the conditions outside the aforementioned ranges using the steels having the chemical composition in the aforementioned ranges.
Note that any of the steel rails listed in Tables 31 and 32, and 33 and 34 were produced under the conditions of a time period of 180 sec. from hot rolling to heat treatment at the railhead portion and an area reduction ratio of 6% at the final pass of finish hot rolling.
In each of those rails, the number of the pearlite blocks having grain sizes in the range from 1 to 15 μm at a portion 5 mm in depth from the head top portion was within the range from 200 to 500 per 0.2 mm2 of observation field.
As seen in Tables 31 and 32, and 33 and 34, in the steel rails having high carbon contents as listed in Table 30, in the cases of the steel rails produced by the heat treatment method according to the present invention wherein the cooling rate at a head inner portion (ICR) was controlled so as to be not lower than the value of CCR calculated from the chemical composition of a steel rail, in contrast to the cases of the rails produced by the comparative heat treatment methods, the formation of pro-eutectoid cementite structures at a head inner portion was prevented and resistance to internal fatigue damage was improved.
In addition, as seen also in Tables 31 and 32, and 33 and 34, it was made possible to prevent the pro-eutectoid cementite structures detrimental to the occurrence of fatigue damage from forming at a head inner portion and, at the same time, to prevent the bainite and martensite structures detrimental to wear resistance from forming in the surface layer of a railhead portion as a result of controlling the value of TCR calculated from the cooling rates at the different positions on the surface of the railhead portion within the range defined by the value of CCR with intent to prevent the formation of pro-eutectoid cementite structures at a railhead inner portion, or secure the cooling rate at a head inner portion (ICR), and stabilize the pearlite structures in the surface layer of a railhead portion.
As described above, in the steel rails having high carbon contents, it was made possible to prevent pro-eutectoid cementite structures detrimental to the occurrence of fatigue damage from forming at a railhead inner portion and, at the same time, obtain pearlite structures highly resistant to wear in the surface layer of a railhead portion as a result of controlling the cooling rate at the railhead inner portion (ICR) within the prescribed range and the cooling rates at the different positions on the surface of the railhead portion within the prescribed range.
TABLE 30
Chemical composition (mass %)
Mo/V/Nb/B/Co/Cu
Steel
C
Si
Mn
Cr
Ni/Ti/Mg/Ca/Al/Zr
79
0.86
0.25
1.15
0.12
80
0.90
0.25
1.21
0.05
Mo: 0.02
81
0.95
0.51
0.78
0.22
82
1.00
0.42
0.68
0.25
83
1.01
0.75
0.35
0.75
Ti: 0.0150
B: 0.0008
84
1.11
0.11
0.31
0.31
Zr: 0.0017
Ca: 0.0021
85
1.19
1.25
0.15
0.15
V: 0.02
Al: 0.08
86
1.35
1.05
0.25
0.25
TABLE 31
Heat
treatment
conditions
Time up
of head
to the
inner
start of
portion
heat
Cooling
treatment
rate *2
Rolling
of head
(value of
length
portion
ICR)
Symbol
Steel
Value of CCR *1
2 CCR
4 CCR
(m)
(sec)
(° C./sec)
Invented
174
79
0.04
0.08
0.16
198
198
0.21
heat
175
80
0.39
0.78
1.56
185
178
0.41
treatment
176
81
0.81
1.62
3.24
185
165
0.91
method
177
81
0.81
1.62
3.24
175
150
1.05
178
82
1.24
2.48
4.96
160
135
1.45
179
82
1.24
2.48
4.96
160
120
1.74
Heat treatment conditions of head surface
Cooling rate
Cooling rate
Cooling rate
at head top
at head side
at lower chin
portion *3 T
portion *3 S
portion *3 A
Value of
Symbol
(° C./sec)
(° C./sec)
(° C./sec)
TCR *4
Microstructure *5
Invented
174
0.5
0.5
0.1
0.13
Head top
Pearlite
heat
portion
treatment
Head inner
Pearlite
method
portion
175
3.0
3.0
1.0
0.95
Head top
Pearlite
portion
Head inner
Pearlite
portion
176
4.0
3.0
3.0
2.00
Head top
Pearlite
portion
Head inner
Pearlite
portion
177
6.0
4.0
4.0
2.70
Head top
Pearlite
portion
Head inner
Pearlite
portion
178
5.0
6.0
5.0
3.35
Head top
Pearlite
portion
Head inner
Pearlite
portion
179
5.0
5.0
6.0
3.75
Head top
Pearlite
portion
Head inner
Pearlite
portion
*1 CCR (° C./sec.) = 0.6 + 10 × ([% C] − 0.9) − 5 × ([% C] − 0.9) × [% Si] − 0.17[% Mn] − 0.13[% Cr]
*2 Cooling rate (° C./sec.) at head inner portion: cooling rate at a depth of 30 mm from head top surface in temperature range from 750° C. to 650° C.
*3 Cooling rates at head surface (head top portion, head side portion and lower chin portion): cooling rate in the region from surface to 5 mm in depth in temperature range from 750° C. to 500° C. Cooling rates at head side portion and lower chin portion are average figures of right and left sides of rail.
*4 TCR = 0.05 × T (cooling rate at head top portion, ° C./sec.) + 0.10 × S (cooling rate at head side portion, ° C./sec.) + 0.50 × J (cooling rate at lower chin portion, ° C./sec.)
*5 Microstructures are observed at a depth of 2 mm (head top portion) and at a depth of 30 mm (head inner portion) from head top surface.
TABLE 32
Heat
treatment
conditions
Time up
of head
to the
inner
start of
portion
heat
Cooling
treatment
rate *2
Rolling
of head
(value of
length
portion
ICR)
Symbol
Steel
Value of CCR *1
2 CCR
4 CCR
(m)
(sec)
(° C./sec)
Invented
180
83
1.13
2.26
4.52
155
110
1.25
heat
181
83
1.13
2.26
4.52
145
80
1.50
treatment
182
84
2.49
4.98
9.97
130
65
3.54
method
183
85
1.64
3.28
6.56
105
35
2.25
184
86
2.66
5.32
10.64
120
15
2.25
Heat treatment conditions of head surface
Cooling rate
Cooling rate
Cooling rate
at head top
at head side
at lower chin
portion *3 T
portion *3 S
portion *3 A
Value of
Symbol
(° C./sec)
(° C./sec)
(° C./sec)
TCR *4
Microstructure *5
Invented
180
6.0
2.0
5.0
3.00
Head top
Pearlite
heat
portion
treatment
Head inner
Pearlite
method
portion
181
8.0
4.0
5.0
3.30
Head top
Pearlite
portion
Head inner
Pearlite
portion
182
6.0
8.0
12.0
7.10
Head top
Pearlite
portion
Head inner
Pearlite
portion
183
4.0
6.0
8.0
4.80
Head top
Pearlite
portion
Head inner
Pearlite
portion
184
12.0
8.0
14.0
8.40
Head top
Pearlite
portion
Head inner
Pearlite
portion
*1 CCR (° C./sec.) = 0.6 + 10 × ([% C] − 0.9) − 5 × ([% C] − 0.9) × [% Si] − 0.17[% Mn] − 0.13[% Cr]
*2 Cooling rate (° C./sec.) at head inner portion: cooling rate at a depth of 30 mm from head top surface in temperature range from 750° C. to 650° C.
*3 Cooling rates at head surface (head top portion, head side portion and lower chin portion): cooling rate in the region from surface to 5 mm in depth in temperature range from 750° C. to 500° C. Cooling rates at head side portion and lower chin portion are average figures of right and left sides of rail.
*4 TCR = 0.05 × T (cooling rate at head top portion, ° C./sec.) + 0.10 × S (cooling rate at head side portion, ° C./sec.) + 0.50 × J (cooling rate at lower chin portion, ° C./sec.)
*5 Microstructures are observed at a depth of 2 mm (head top portion) and at a depth of 30 mm (head inner portion) from head top surface.
TABLE 33
Heat
treatment
conditions
Time up
of head
to the
inner
start of
portion
heat
Cooling
treatment
rate *2
of head
(value of
portion
ICR)
Symbol
Steel
Value of CCR *1
2 CCR
4 CCR
Rolling length (m)
(sec)
(° C./sec)
Comparative
185
80
0.39
0.78
1.56
198
198
0.30
heat
(Insufficient
treatment
cooling)
method
186
80
0.39
0.78
1.56
185
178
1.25
187
81
0.81
1.62
3.24
185
165
0.55
(Insufficient
cooling)
188
81
0.81
1.62
3.24
175
150
1.75
189
82
1.24
2.48
4.96
160
135
1.05
(Insufficient
cooling)
190
82
1.24
2.48
4.96
160
120
2.35
Heat treatment conditions of head surface
Cooling rate
Cooling rate
Cooling rate
at head top
at head side
at lower chin
portion *3 T
portion *3 S
portion *3 A
Value of
Symbol
(° C./sec)
(° C./sec)
(° C./sec)
TCR *4
Microstructure *5
Comparative
185
2.0
1.0
1.0
0.70
Head top
Pearlite
heat
(Insufficient
portion
treatment
cooling)
Head inner
Pearlite + pro-
method
portion
eutectoid
cementite
186
6.0
5.0
4.0
2.80
Head top
Pearlite + bainite +
(Over-
portion
martensite
cooling)
Head inner
Pearlite
portion
187
4.0
1.0
2.0
1.30
Head top
Pearlite
(Insufficient
portion
cooling)
Head inner
Pearlite + pro-
portion
eutectoid
cementite
188
5.0
5.0
6.0
3.75
Head top
Pearlite
(Over-
portion
cooling)
Head inner
Pearlite + bainite +
portion
martensite
189
4.0
4.0
3.0
2.10
Head top
Pearlite
(Insufficient
portion
cooling)
190
10.0
10.0
7.0
5.00
Head inner
Pearlite + pro-
(Over-
portion
eutectoid
cooling)
cementite
*1 CCR (° C./sec.) = 0.6 + 10 × ([% C] − 0.9) − 5 × ([% C] − 0.9) × [% Si] − 0.17[% Mn] − 0.13[% Cr]
*2 Cooling rate (° C./sec.) at head inner portion: cooling rate at a depth of 30 mm from head top surface in temperature range from 750° C. to 650° C.
*3 Cooling rates at head surface (head top portion, head side portion and lower chin portion): cooling rate in the region from surface to 5 mm in depth in temperature range from 750° C. to 500° C. Cooling rates at head side portion and lower chin portion are average figures of right and left sides of rail.
*4 TCR = 0.05 × T (cooling rate at head top portion, ° C./sec.) + 0.10 × S (cooling rate at head side portion, ° C./sec.) + 0.50 × J (cooling rate at lower chin portion, ° C./sec.)
*5 Microstructures are observed at a depth of 2 mm (head top portion) and at a depth of 30 mm (head inner portion) from head top surface.
TABLE 34
Heat
treatment
conditions
Time up
of head
to the
inner
start of
portion
heat
Cooling
treatment
rate *2
of head
(value of
Rolling
portion
ICR)
Symbol
Steel
Value of CCR *1
2 CCR
4 CCR
length (m)
(sec)
(° C./sec)
Comparative
191
82
1.24
2.48
4.96
160
250
2.20
heat
(Time too
treatment
long,
method
cementite
formed)
192
83
1.13
2.26
4.52
145
80
0.95
(Insufficient
cooling)
193
84
2.49
4.98
9.97
130
65
1.00
(Insufficient
cooling)
194
86
2.66
5.32
10.64
245
15
2.25
(Excessive
rail length, rail
ends
overcooled)
Heat treatment conditions of head surface
Cooling rate
Cooling rate
Cooling rate
at head top
at head side
at lower chin
portion*3 T
portion *3 S
portion *3 A
Value of
Symbol
(° C./sec)
(° C./sec)
(° C./sec)
TCR *4
Microstructure *5
Comparative
191
4.0
5.0
6.0
3.70
Head top
Pearlite
heat
portion
treatment
Head inner
Pearlite + trace
method
portion
pro-
eutectoid
cementite
192
6.0
2.0
3.0
2.00
Head top
Pearlite
(Insufficient
portion
cooling)
Head inner
Pearlite + pro-
portion
eutectoid
cementite
193
4.0
4.0
3.0
2.10
Head top
Pearlite
(Insufficient
portion
cooling)
Head inner
Pearlite + pro-
portion
eutectoid
cementite
194
12.0
8.0
14.0
8.40
Head top
Pearlite
portion
Head inner
Pearlite + trace
portion
pro-
eutectoid
cementite
*1 CCR (° C./sec.) = 0.6 + 10 × ([% C] − 0.9) − 5 × ([% C] − 0.9) × [% Si] − 0.17[% Mn] − 0.13[% Cr]
*2 Cooling rate (° C./sec.) at head inner portion: cooling rate at a depth of 30 mm from head top surface in temperature range from 750° C. to 650° C.
*3 Cooling rates at head surface (head top portion, head side portion and lower chin portion): cooling rate in the region from surface to 5 mm in depth in temperature range from 750° C. to 500° C. Cooling rates at head side portion and lower chin portion are average figures of right and left sides of rail.
*4 TCR = 0.05 × T (cooling rate at head top portion, ° C./sec.) + 0.10 × S (cooling rate at head side portion, ° C./sec.) + 0.50 × J (cooling rate at lower chin portion, ° C./sec.)
*5 Microstructures are observed at a depth of 2 mm (head top portion) and at a depth of 30 mm (head inner portion) from head top surface.
The present invention makes it possible to provide: a pearlitic steel rail wherein the wear resistance required of the head portion of a rail for a heavy load railway is improved, rail breakage is inhibited by controlling the number of fine pearlite block grains at the railhead portion and thus improving ductility and, at the same time, toughness of the web and base portions of the rail is prevented from deteriorating by reducing the amount of pro-eutectoid cementite structures forming at the web and base portions; and a method for efficiently producing a high-quality pearlitic steel rail by optimizing the heating conditions of a bloom (slab) for the rail and, by so doing, preventing the generation of cracks and breaks during hot rolling, and suppressing decarburization at the outer surface of the bloom (slab).
Kobayashi, Akira, Fujita, Kazuo, Ueda, Masaharu, Morohoshi, Takashi, Matsushita, Koichiro, Uchino, Koichi, Iwano, Katsuya
| Patent | Priority | Assignee | Title |
| 10113219, | Jun 24 2014 | YANSHAN UNIVERSITY | Nano-pearlite rail and process for manufacturing same |
| 11401591, | Dec 15 2015 | JFE Steel Corporation | Method for selecting rail steel and wheel steel |
| 8210019, | Jul 24 2006 | Nippon Steel Corporation | Method for producing pearlitic rail excellent in wear resistance and ductility |
| 8469284, | Feb 18 2009 | Nippon Steel Corporation | Pearlitic rail with excellent wear resistance and toughness |
| 8747576, | Jun 26 2009 | Nippon Steel Corporation | Pearlite-based high carbon steel rail having excellent ductility and process for production thereof |
| 9670570, | Apr 17 2014 | EVRAZ INC. NA CANADA; EVRAZ INC NA CANADA | High carbon steel rail with enhanced ductility |
| Patent | Priority | Assignee | Title |
| 20010025674, | |||
| CA2154779, | |||
| EP685566, | |||
| EP7547751, | |||
| EP685566, | |||
| JP11092867, | |||
| JP2001234238, | |||
| JP2002226914, | |||
| JP2002226915, | |||
| JP8049019, | |||
| JP9137227, | |||
| JP9137228, | |||
| JP9316598, |
| Executed on | Assignor | Assignee | Conveyance | Frame | Reel | Doc |
| Jul 19 2007 | Nippon Steel Corporation | (assignment on the face of the patent) | / |
| Date | Maintenance Fee Events |
| Jul 10 2012 | ASPN: Payor Number Assigned. |
| Dec 17 2014 | M1551: Payment of Maintenance Fee, 4th Year, Large Entity. |
| Dec 20 2018 | M1552: Payment of Maintenance Fee, 8th Year, Large Entity. |
| Feb 20 2023 | REM: Maintenance Fee Reminder Mailed. |
| Aug 07 2023 | EXP: Patent Expired for Failure to Pay Maintenance Fees. |
| Date | Maintenance Schedule |
| Jul 05 2014 | 4 years fee payment window open |
| Jan 05 2015 | 6 months grace period start (w surcharge) |
| Jul 05 2015 | patent expiry (for year 4) |
| Jul 05 2017 | 2 years to revive unintentionally abandoned end. (for year 4) |
| Jul 05 2018 | 8 years fee payment window open |
| Jan 05 2019 | 6 months grace period start (w surcharge) |
| Jul 05 2019 | patent expiry (for year 8) |
| Jul 05 2021 | 2 years to revive unintentionally abandoned end. (for year 8) |
| Jul 05 2022 | 12 years fee payment window open |
| Jan 05 2023 | 6 months grace period start (w surcharge) |
| Jul 05 2023 | patent expiry (for year 12) |
| Jul 05 2025 | 2 years to revive unintentionally abandoned end. (for year 12) |